Method of and apparatus for encoding and decoding data

ABSTRACT

When encoding a texture map  1  for use in graphics processing, the texture map is divided into a plurality of equal-sized blocks  2  of texture data elements. Each block  2  of texture data elements is then encoded as a block of texture data  5  that includes a set of integer values to be used to generate a set of base data values for the block, and a set of index values indicating how to use the base data values to generate data values for the texture data elements that the block represents. The integer values and the index values are both encoded in an encoded texture data block using a combination of base-n values, where n is greater than two, and base-2 values. Predefined bit representations are used to represent plural base-n values (n&gt;2) collectively, and the bits of the bit representations representing the base-n values (n&gt;2) are interleaved with bits representing the base-2 values in the encoded texture data block.

CLAIM OF PRIORITY

This application is a divisional of U.S. patent application Ser. No.13/464,663, “Method of and Apparatus for Encoding and Decoding Data”filed on May 4, 2012, which claims priority to UK Application No.1107523.1 filed May 5, 2011 and UK Application No. 1118037.9 filed Oct.19, 2011, all of which are incorporated herein by reference in theirentirety.

BACKGROUND

The technology described herein relates to a method of and apparatus forencoding and decoding data, and in particular to such a method andapparatus for use to compress and decompress texture data in computergraphics systems.

It is common in computer graphics systems to generate colours forsampling positions in the image to be displayed by applying so-calledtextures or texture data to the surfaces to be drawn. For example,surface detail on objects may be generated by applying a predefined“texture” to a set of polygons representing the object, to give therendered image of the object the appearance of the “texture”. Suchtextures are typically applied by storing an array of texture elementsor “texels”, each representing given texture data (such as colour,luminance, and/or light/shadow, etc. values), and then mapping thetexels onto the corresponding elements, such as (and, indeed, typically)a set of sampling positions, for the image to be displayed. The storedarrays of texture elements (data) are typically referred to as “texturemaps”.

Such arrangements can provide high image quality, but have a number ofdrawbacks. In particular, the storage of the texture data and accessingit in use can place, e.g., high storage and bandwidth requirements on agraphics processing device (or conversely lead to a loss in performancewhere such requirements are not met). This is particularly significantfor mobile and handheld devices that perform graphics processing, assuch devices are inherently limited in their, e.g., storage, bandwidthand power resources and capabilities.

It is known therefore to try to encode such texture data in a“compressed” form so as to try to reduce, e.g., the storage andbandwidth burden that may be imposed on a device.

One known such texture data compression technique determines a set orpalette of colours to be used for, e.g., a given texture map, and thenstores for each texture element (texel) in the texture map an index intothe set or palette of colours, indicating the colour to be used for thattexel. This has the advantage that only an index, rather than a full(e.g.) colour value needs to be stored for each texel. This helps toreduce, e.g., the texture data storage requirements, but still has somedrawbacks, such as in terms of reduced image quality and the necessarydata processing.

Another known texture compression technique is to use so-called blocktruncation coding (BTC). In this technique the overall texture array(texture map) is subdivided into smaller blocks, e.g. of 4×4 texels, anda number (typically two) of base or primary colour values are determinedfor each such block, with each texel in the block being set to one ofthe base colour values. This again saves on the data that has to bestored and accessed, but at a cost of lower image quality.

U.S. Pat. No. 5,047,853 describes an improved block truncation codingtechnique. In this technique, two base colours are again stored for eachtexel block, but two additional colours to be used for the block arealso derived from those two base colours (e.g. by linearly blendingthose colours). In this way, four colours are provided as a “palette”for the texel block, but only two colour values need to be stored forthe block. Each texel in the block is then encoded using two bits, toindicate which of the four block “colours” should be used for the texel.This system provides improved image quality over basic block truncationcoding, but requires more data per block.

In texture compression schemes, each encoded texture data block willtypically include, inter alia, one or more integer values, representing,e.g., data values such as endpoint or base colours, to be used whendetermining the data value (e.g. colour) for a texture data element thatthe block represents. The encoded block may also include other integervalues, such as index values for texture data elements that the blockrepresents. While these integer values can be encoded in an encodedtexture data block in any desired fashion, the Applicants haverecognised that there is scope for improved integer value encoding andcompression techniques for use when, e.g., encoding texture data forgraphics processing.

BRIEF DESCRIPTION OF THE DRAWINGS

A number of embodiments of the technology described herein will now bedescribed by way of example only and with reference to the accompanyingdrawings, in which:

FIG. 1 shows schematically the encoding of an array of image data as aplurality of encoded data blocks in accordance with an embodiment of thetechnology described herein;

FIG. 2 shows schematically the encoding of a partitioned data block inthe described embodiment of the technology described herein;

FIGS. 3 to 11 show encoded block layouts of the described embodiment ofthe technology described herein; and

FIG. 12 shows schematically a graphics processing system that can usetexture data that has been encoded in the manner of the describedembodiment of the technology described herein.

DETAILED DESCRIPTION

A first embodiment of the technology described herein comprises a methodof representing integer values to be encoded in an encoded texture datablock that represents a set of graphics texture data elements to be usedin a graphics processing system, the method comprising:

representing an integer value to be encoded in the encoded texture datablock using a base-n value, where n is greater than two.

A second embodiment of the technology described herein comprises anapparatus for representing integer values to be encoded in an encodedtexture data block that represents a set of graphics texture dataelements to be used in a graphics processing system, the apparatuscomprising:

processing circuitry for representing an integer value to be encoded inthe encoded texture data block using a base-n value, where n is greaterthan two.

Another embodiment of the technology described herein comprises a blockof texture data representing a set of texture data elements to be usedin a graphics processing system, wherein:

one or more integer values encoded in the texture data block arerepresented using a base-n value, where n is greater than two.

The technology described herein uses base-n values, where n is greaterthan two, to represent integer values to be included in an encodedtexture data block when encoding texture data for use in a graphicsprocessing system.

Using base-n (n>2) values to represent integer values in an encodedtexture data block helps to make the encoding more efficient, as it can,for example, allow the use of non-integer numbers of bits for encodinginteger values in an encoded texture data block. It can also, as will bediscussed further below, facilitate a particularly flexible, andreadily, and finely, adjustable, integer value encoding scheme.

Any base-n values (n>2) can be used to encode the integer values.However in an embodiment the encoding can use and uses base-3 values,which are in an embodiment encoded using trits (items that can takethree values, namely the values 0, 1, 2 (whereas base-2 values areencoded using bits, which can only take the values 0 and 1)). In anembodiment the encoding can also or instead use, and uses, base-5values, which are in an embodiment encoded using quints (items that cantake five values, namely the values 0, 1, 2, 3, 4). In an embodimentboth base-3 and base-5 values can be, and are, used to encode integervalues in the encoded texture data blocks.

In an embodiment, base-2 values can also be used to encode integervalues in an encoded texture data block. Thus in an embodiment acombination of both base-2 values, and base-n values (where n is greaterthan two) can be, and are, used to represent integer values when theyare encoded. Accordingly, in an embodiment, the integer values encodedin an encoded texture data block can be, and are, encoded using acombination of base-2 values, and base-3 and/or base-5 values. In anembodiment a combination of base-2, base-3 and base-5 values can be, andare, used.

The base-n (n>2) values used to represent the integer values to beencoded can be encoded in the encoded texture data block in any desiredand suitable manner. However, in an embodiment the base-n (n>2) valuesare stored (encoded) in the encoded texture data block using predefinedbit representations (with each respective bit representation indicatinga given base-n value (n>2) or set of plural base-n values (n>2)).

In an embodiment, bit representations that each represent a particularcombination of plural base-n values (e.g., and in an embodiment, base-n(n>2) values for a group of plural integer values to be encoded in theencoded texture data block) are used, as this has been found to providea particularly efficient way of storing the base-n values in the encodedtexture data block.

For example, in an embodiment the bit representations that are used fortrits are such that n trits will be represented with

$\lceil \frac{8n}{5} \rceil$bits (i.e. bit representations of 8 bits are in an embodiment used toindicate the values of 5 trits). Similarly, for quints, the bitrepresentations are in an embodiment such that n quints will berepresented with

$\lceil \frac{7n}{3} \rceil$bits (i.e. bit representations of 7 bits are used to indicate the valuesof 3 quints).

It is believed that the use of such bit representations may be new andadvantageous in its own right.

Thus an embodiment of the technology described herein comprises a methodof representing base-n values, where n is greater than two, to be usedto represent integer values to be encoded in an encoded texture datablock that represents a set of graphics texture data elements to be usedin a graphics processing system, the method comprising:

using predefined bit representations to represent collectively pluralbase-n values, where n is greater than two, in an encoded texture datablock.

Another embodiment of the technology described herein comprises anapparatus for representing base-n values, where n is greater than two,to be used to represent integer values to be encoded in an encodedtexture data block that represents a set of graphics texture dataelements to be used in a graphics processing system, the apparatuscomprising:

processing circuitry for using predefined bit representations torepresent collectively plural base-n values, where n is greater thantwo, in an encoded texture data block.

An embodiment of the technology described herein comprises a block oftexture data representing a set of texture data elements to be used in agraphics processing system, wherein:

plural integer values encoded in the texture data block are representedusing base-n values, where n is greater than two; and

the block includes:

a predefined bit representation that represents collectively a pluralityof the base n-values.

As will be appreciated by those skilled in the art, these embodiments ofthe technology described herein can, and in an embodiment do, includeany one or more or all of the preferred and optional features of thetechnology described herein described herein, as appropriate.

The bit representations for the sets of plural base-n values (n>2) arein an embodiment representations from which the individual base n (n>2)values (in the set) can be derived by using bit manipulation of the bitrepresentation (and in an embodiment bit manipulations that can easilybe implemented in hardware). This then facilitates decoding the bitrepresentations more straightforwardly in hardware. In an embodiment,particular bit patterns (representations) are used to represent anyspecial cases (combinations of base-n values) that are difficult torepresent in a form that can easily be decoded using bit manipulation.

Other arrangements, such as the use of look-up tables, to interpret thebit representations would, of course, be possible.

Where an integer value is to be encoded using a base-n (n>2) value, thenin an embodiment each integer value to be encoded is broken into plural,and in an embodiment two, parts before encoding, and in an embodimentinto respective high and low parts (portions). One of these parts (in anembodiment the low part) is in an embodiment then represented by zero ormore bits (base-2 values), and the other part (in an embodiment the highpart) by one or more (and in an embodiment by one) base-n (n>2) values(e.g., and in an embodiment, trit(s) or quint(s)).

Thus, in a embodiment, at least some of the integer values included inthe encoded texture data block are encoded using a combination of one ormore base-2 values and one or more base-n (n>2) values (and in anembodiment one and only one base-n (n>2) value). Similarly, thetechnology described herein in an embodiment comprises encoding one ormore of the integer values included in the encoded texture data blockusing a combination of one or more base-2 values and one or more (and inan embodiment one and only one) base-n (n>2) values. In thesearrangements, the base-2 values in an embodiment represent the lowerpart (portion) of the integer value, and the base-n (n>2) value(s) thehigher part (portion).

Where the integer values to be encoded in the encoded texture data blockare represented using a combination of base-n (n>2) values and bits(base-2 values), then the bit representations for the base-n (n>2)values and the bit values being used to encode the integer values in theencoded texture data block can be organised in the encoded texture datablock in any desired and suitable manner. For example, the bitrepresentations for the base-n (n>2) values could be placed together,with the bit values for the base-2 values then following (orvice-versa).

However, in an embodiment, where the integer values to be encoded in theencoded texture data block are represented using a combination of base-n(n>2) values and bits (base-2 values), then in an embodiment the bits ofthe bit representations for the base-n (n>2) values are interleaved withthe bits for the base-2 values in the encoded texture data block.Accordingly, where the base-n (n>2) values for a group of integers to beencoded are represented as a particular bit representation (as discussedabove), the bits of that bit representation are in an embodimentinterleaved with the bits for the base-2 values for that group ofintegers.

In an embodiment the bits for the base-2 values for a given integer areplaced next to the bits of the base-n (n>2) value bit representationthat represent or indicate the base n (n>2) value for that integer. Inother words, the base-n (n>2) value information (bits) for a giveninteger value being encoded are in an embodiment placed next to thebase-2 value information for that integer value in the encoded texturedata block.

It is again believed that such arrangements may be new and advantageousin their own right.

Thus, an embodiment of the technology described herein comprises amethod of encoding integer values to be encoded in an encoded texturedata block that represents a set of graphics texture data elements to beused in a graphics processing system, the method comprising:

representing a set of integer values to be encoded in the encodedtexture data block using a combination of base-n values, where n isgreater than two, and base-2 values;

representing the values of the base-n values (n>2) for the set ofinteger values using bit representations, and representing the values ofthe base-2 values for the set of integer values using bits; and

interleaving the bits of the bit representations representing the valuesof the base-n values (n>2) with the bits representing the values of thebase-2 values in the encoded texture data block.

An embodiment of the technology described herein comprises an apparatusfor encoding integer values to be encoded in an encoded texture datablock that represents a set of graphics texture data elements to be usedin a graphics processing system, the apparatus comprising:

processing circuitry for representing a set of integer values to beencoded in the encoded texture data block using a combination of base-nvalues, where n is greater than two, and base-2 values;

processing circuitry for representing the values of the base-n values(n>2) for the set of integer values using bit representations, andrepresenting the values of the base-2 values for the set of integervalues using bits; and

processing circuitry for interleaving the bits of the bitrepresentations representing the values of the base-n values (n>2) withthe bits representing the values of the base-2 values in the encodedtexture data block.

An embodiment of the technology described herein comprises a block oftexture data representing a set of texture data elements to be used in agraphics processing system, wherein:

the block of texture data:

represents a set of integer values encoded in the encoded texture datablock using a combination of base-n values, where n is greater than two,and base-2 values; and

represents the values of the base-n values (n>2) for the set of integervalues using bit representations, and representing the values of thebase-2 values for the set of integer values using bits; and wherein:

the bits of the bit representations representing the values of thebase-n values (n>2) are interleaved with the bits representing thevalues of the base-2 values in the encoded texture data block.

As will be appreciated by those skilled in the art, these embodiments ofthe technology described herein can, and in an embodiment do, includeany one or more or all of the preferred and optional features of thetechnology described herein described herein, as appropriate.

In a embodiment, the encoding process is configured to store the“useful” information in the lowest bits in the relevant sequence ofencoded bits, and if there are not enough useful values to fill theentire sequence of bits that would be encoded, the bit sequence ispadded with zeroes up to the required size at encode time. The decoderis correspondingly in an embodiment configured to assume that any bitsthat are “missing” from the encoded texture data block in a sequence ofbits encoding integer values are always zero. This means that if theencoding is done in a defined order (which it in an embodiment is), the“dummy”, padding zeroes do not need to be included in the encodedtexture data block, thereby facilitating more efficient inclusion of theinteger values in the encoded texture data block.

Where a set of integer values is to be encoded using base-2 values (i.e.bits) only (as will be discussed further below), then in an embodimentthe integers are encoded sequentially in the encoded texture data block,with lowest bit appearing first in the sequence encoding.

The integer value encoding arrangement of the technology describedherein can be used to encode any desired integer values that are to beincluded in an encoded texture data block, such as colour (or other)data values to be included in the block, index values to be included inthe block, etc.

In an embodiment, the encoding is used to encode sequences or sets ofone or more, and in an embodiment of plural, integer values in theencoded texture data block. In an embodiment, plural sets of integervalues are encoded in the encoded texture data block(s) in the manner ofthe technology described herein. In one embodiment, the encoded texturedata block includes two sequences or sets of integer values, that areeach respectively encoded in the manner of the technology describedherein.

In an embodiment the integer values that are encoded in the manner ofthe technology described herein comprise both index values for use forthe encoded texture data block, and one or more data values (e.g. colourvalues) to be used as, and/or to generate, a set of base data values(e.g., and in an embodiment, endpoint colour values) to be used for theencoded texture data block.

In an embodiment some or all of the integer values that are encoded inthe block are integer values from a defined (allowable) range or rangesof integer values. Thus, a given encoded texture data block in anembodiment can have defined for it a range (or ranges) of integervalues, from which a set of integer values for the block are then taken(i.e. such that each integer value in the set will be an integer fromthe defined (permitted) integer value range).

In this case the range(s) of values that the integer values fall withincan in an embodiment be defined for, and can be and are derived for,each encoded texture data block separately (i.e. on a block-by-blockbasis).

This will then allow, e.g., some encoded blocks to use a greater rangeof integer values, and other encoded texture data blocks to use a morerestricted range of integer values. This further enhances theflexibility of the system of the technology described herein.

In an embodiment a number of different integer value ranges can bedefined and are supported (i.e. the integer values to be encoded canextend over different ranges of permitted values).

In an embodiment, the integer value encoding arrangement of thetechnology described herein is used to encode a set or group, and in anembodiment a sequence, of integers within (from) a bounded range, suchas a set of index values, a set of data (e.g. colour) values, etc. In anembodiment plural sets (sequences) of integer values are encoded, suchas, and in an embodiment, one set representing a set of index values forthe encoded block, and another set representing a set of data values forthe block. In this case, each set of integer values is in an embodimentencoded separately to the other set or sets of integer values, and eachset of integer values can in an embodiment have its own defined valuerange that it is constrained to be within.

The range or each range of integer values could, e.g., comprise the“true” range over which the values in question (e.g. colour values) needto be represented. However, in an embodiment, the integer values thatare used (and encoded in the encoded texture data block) are taken froma constrained range of values, which values are then “expanded” or“blown-up” to corresponding values from the “true” range that the valuesin question (e.g. colour values) can take (on decoding). In other words,the integer values which are encoded are in an embodiment constrained tobe from a restricted range of permitted values, which values are thenconverted to corresponding, “true”, values as part of the decodingprocess.

In these arrangements, the permitted (defined) integer value range willdetermine how many different values from the overall, “true” range forthe values in question can be indicated by the encoded texture datablock (and their spacing across the overall, “true” range for thevalues).

This could be done in respect of some or all of the groups of integervalues that are to be encoded. It is in an embodiment done at least inrespect of a group or groups of integer values that are to representdata values (such as colour values) and/or index values to be used forthe block.

Where the encoding schemes uses defined ranges of integer values, thenin an embodiment, the encoding scheme that is used for the integervalues in a given set of or sequence of integer values is dependent uponthe defined range that the sequence of integer values is taken from(bounded to be within). As discussed above, in an embodiment a pluralityof allowed (supported) integer value ranges are defined, and each rangein an embodiment has its own predefined encoding scheme.

In an embodiment the integer values of an integer value range areencoded using just bits (base-2 values) if that is the most efficientmanner to encode them, i.e. where the number of integer values in therange is 2^(n), but are encoded using bits (base-2 values) and a base-nvalue (where n is greater than 2) otherwise. In an embodiment theencoding is such that for any given range of integer values at most onebase-n value (where n is greater than 2), e.g. one trit or one quint,and then however many base-2 values (bits) are required to represent thevalues in the range in combination with the base-n value (n>2), are usedto encode the integer values (each integer value) taken from that range.Thus, in an embodiment, each integer value is represented for encodingpurposes using one of: a number of bits; one trit; one quint; a numberof bits and one trit; or a number of bits and one quint, in anembodiment depending upon the range from which the integer value istaken (the range that the integer value is constrained to be within).

The Applicants have found that this arrangement can provide a canonicalrepresentation of integer values, that is easy to use, for pluraldifferent ranges of integer values. Also, it can allow the number ofbits required to encode different sets of integer values (such as dataand index values) in the encoded texture data block to be adjusted in afine-grained manner (as increasing an integer value range being used byone, for example, may only effectively require one more bit, or indeed,less than a bit, per individual value in the encoded texture datablock). This fine-grained adjustment can then, e.g., allow thetrading-off of encoding bits between different sets of integer values tobe encoded in a block, such as, e.g., and in an embodiment, between theindex values and the (e.g. endpoint) colour values that are included inan encoded texture data block, e.g., on a block-by-block basis. Thisaccordingly can provide a very flexible, but still efficient, encodingsystem.

It will be appreciated from the above that the integer value encodingscheme of the technology described herein is accordingly in anembodiment used to encode (store) sets (sequences) of integers fromwithin respective bounded ranges in an encoded texture data block, withthe number of base-2 values and base-n (n>2) values (e.g. trits orquints) being used to encode (store) each integer being determined bythe range that the respective set (sequence) of integers is constrainedto.

In one embodiment the defined range that has been used for a set ofinteger values is indicated explicitly in the encoded texture data block(this is in an embodiment done where the set of integer valuesrepresents index values). This avoids the need to explicitly encode therange in the encoded data block. In another embodiment, this is notdone, and instead the defined range is derived implicitly from theencoded texture data block. This is in an embodiment done where the setof integer values indicate data, e.g., endpoint colour, values to beused for the block.

In an embodiment, at least one set of integer values has its rangeindicated explicitly in the encoded texture data block, but another setof integer values does not (i.e. its defined range must be derivedimplicitly from the encoded texture data block).

Where the range used for a set or sequence of integer values is to bederived implicitly from an encoded texture data block, then this is inan embodiment done by determining the available space in the encodedtexture data block for encoding the set of integer values, and thendetermining the range used based on that available space and the numberof integer values in the set (i.e. that must actually be encoded in theavailable space) (and, e.g., and in an embodiment, based on the integervalue encoding scheme being used). The range used is in an embodimentdetermined as being the largest integer value range whose encodingscheme (format) will still permit the required number of integer valuesto be included in the encoded block to fit in the available space.

This determination is in an embodiment done when decoding the texturedata block (to determine the range that has been used for the integervalues when encoding the block), and is in an embodiment also done whenencoding the encoded texture data block (to determine the range shouldbe used for the integer values for the block). As discussed above, in anembodiment the largest range of integer values that can be used (whilestill being able to fit the required number of integer values in theencoded block) is used.

It is believed that such arrangements are particularly advantageous, asthey facilitate, e.g., a more flexible system, but which can stillalways use the highest resolution available for the integer values forany given encoded texture data block.

Thus, an embodiment of the technology described herein comprises amethod of encoding a set of texture data elements to be used in agraphics processing system, comprising:

encoding the set of texture data elements as a block of texture datarepresenting the texture data elements; and

including in the texture data block:

a set of integer values (in an embodiment for use when generating datavalues for a set of the texture data elements that the blockrepresents); wherein:

the integer values are constrained to be from a restricted range ofpermitted integer values; and

the method further comprises:

determining the range to be used for the set of integer values based onthe number of integer values in the set and the space available in theencoded texture data block for encoding the set of integer values.

An embodiment of the technology described herein comprises an apparatusfor encoding a set of texture data elements to be used in a graphicsprocessing system, comprising:

processing circuitry for encoding the set of texture data elements as ablock of texture data representing the texture data elements; and

processing circuitry for including in the texture data block:

a set of integer values (in an embodiment for use when generating datavalues for a set of the texture data elements that the blockrepresents); wherein:

the integer values are constrained to be from a restricted range ofpermitted integer values; and

the apparatus further comprises:

processing circuitry for determining the range to be used for the set ofinteger values based on the number of integer values in the set and thespace available in the encoded texture data block for encoding the setof integer values.

An embodiment of the technology described herein comprises a method ofdecoding a block of encoded texture data representing a set of texturedata elements to be used in a graphics processing system, comprising:

using a set of integer values included in the encoded texture data blockwhen decoding the block (and in an embodiment to generate data valuesfor a set of the texture data elements that the block represents);wherein:

the integer values are constrained to be from a restricted range ofpermitted integer values; and

the method further comprises:

determining the range that has been used for the set of integer valuesbased on the number of integer values in the set and the space availablein the encoded texture data block for encoding the set of integervalues.

An embodiment of the technology described herein comprises an apparatusfor decoding a block of encoded texture data representing a set oftexture data elements to be used in a graphics processing system,comprising:

processing circuitry for using a set of integer values included in theencoded texture data block when decoding block (and in an embodiment togenerate data values for a set of the texture data elements that theblock represents); wherein:

the integer values are constrained to be from a restricted range ofpermitted integer values; and

the apparatus further comprises:

processing circuitry for determining the range that has been used forthe set of integer values based on the number of integer values in theset and the space available in the encoded texture data block forencoding the required integer values.

As will be appreciated by those skilled in the art, these embodiments ofthe technology described herein can, and in an embodiment do, includeany one or more or all of the preferred and optional features of thetechnology described herein described herein, as appropriate. Thus, forexample, in an embodiment the largest range of values for which the setof integer values can be encoded in the block (i.e. in the determinedavailable space in the block) is used. Similarly, in a embodiment, thisarrangement is used for a set of integer values for use to generate aset of data values (e.g. colour values) to be used to generate datavalues for a set of the texture data elements that the encoded block oftexture data represents.

In these embodiments of the technology described herein, the availablespace in the encoded texture data block for encoding the requiredinteger values may be determined as desired. It is in an embodimentdetermined by determining the space (number of bits) required torepresent the other information that must be included in the encodedtexture data block and then subtracting that from the total capacity(bit capacity) of the encoded texture data block.

The decoder thus in an embodiment uses information included in theencoded texture block, such as, and in an embodiment informationindicating the encoding arrangement that has been used for the block, todetermine the space (number of bits) required to represent all theinformation in the block apart from the set of integers whose range isunknown (and in an embodiment then subtracts that bit count from the(known) size of the block to determine the space (number of bits)available for the set of integers whose range is unknown, and thendetermines the range that has been used for the set of integers whoserange is unknown based on the determined available space).

Similarly, the number of integer values in the set (to be included inthe encoded texture data block) may be determined as desired. It is inan embodiment determined from or using information included in theencoded texture data block that indicates how many of the respectiveinteger values are required to decode, and/or are included in, theblock.

The decoder thus in an embodiment uses information included in theencoded texture block, such as, and in an embodiment informationindicating how many of the respective integer values are required todecode and/or are included in, the block, to determine the number ofinteger values in the set of integers whose range is unknown included inthe block.

Although the technology described herein is directed in particular tothe encoding of integer values in an encoded texture data block, it willbe appreciated that the overall encoding process should be such that an(and each) encoded texture data block will include all the informationthat is necessary for, and/or expected by, a decoder, to allow thedecoder to decode the encoded texture data block to reproduce (at leastapproximately) the original set of texture data elements. The data thatshould be included in the encoded texture data block to achieve thiswill depend upon the exact nature of the texture data encoding(compression) scheme in question. This data can be arranged in anysuitable and desired manner in the encoded texture data block (i.e. inpractice in the manner that a decoder can recognise and correctlyinterpret).

In general it is in an embodiment (and in a embodiment this is done) forthe encoded texture data block to include at least data indicating orallowing to be derived a base data (e.g. colour) value or values (suchas colour endpoint values) for the encoded texture data block, andinformation to allow the decoder to determine (at least an approximationto) the value of a given texture data element from the base data valueor values (e.g. endpoint colours) for the block.

In one embodiment, each encoded block of texture data includes, interalia, data indicating, or indicating how to generate, a set of base datavalues (e.g. colour endpoint values) to be used to generate data valuesfor a set of the texture data elements that the block represents, anddata indicating how to use the set of base data values (e.g. colourendpoint values) to generate data values for texture data elements ofthe set of texture data elements that the set of base data values is tobe used for.

The base set of data values (e.g. endpoint colours) to be used for ablock can be generated as desired, for example, by, as is known in theart, assessing the data values present in the original texture data(i.e. the data which is to be encoded and compressed) and derivingtherefrom a set of data values that are representative of and/or can beused to derive, the original data values. Any suitable technique can beused for this, such as, for example, using error comparison techniquesto determine a reduced set of data values that best matches the originalset of data values.

The base set of data values is in an embodiment determined on ablock-by-block basis. For example, in the case of colour data, as inconventional block truncation encoding techniques, one or two (or more)base representative colours could be determined for the texture block,which colours would then serve as the base colour palette to be usedwhen generating the set of colours to be used when reproducing thetexture block.

In an embodiment, the set of base data values for a block comprises apair (or respective pairs) of data values. In an embodiment it comprisesa pair (or pairs) of endpoint data values, such as a pair of endpointcolour values, from which, e.g., data (e.g. colour) values for texturedata elements of the block in an embodiment can be interpolated (and inan embodiment can be derived by determining a weighted sum of theendpoint values for the block). The pairs of base data values couldinstead comprise, e.g., a base value and a difference value that canthen be combined to give values for the texture data elements, ifdesired.

In an embodiment, the encoded texture data blocks include data forindicating how to generate a set or sets of base data values for theblock using a set of integer values that are encoded in the block (whichinteger values are in an embodiment encoded in the block in the mannerof the technology described herein).

In this case, the data value generation method in an embodiment usesplural integer values to derive the base data values, such as, and in anembodiment, two, four, six or eight integer values.

The integer values may be used to generate the base data values to beused in any desired and suitable manner. In one embodiment, the integervalues are used directly to generate the base data values. In this case,one or more integer values will be used for one endpoint value, andanother integer value or values used for the other endpoint value, forexample.

In another embodiment, some or all of the base data values to be usedare derived from the integer values. In this case, the integer valuescan in an embodiment be used as, or to derive, an offset to apply toanother integer value or values to generate one or more of the base datavalues (e.g. endpoint values) to be used, and/or they can in anembodiment be used as, or to derive, a scaling factor to apply toanother integer value or values to generate one (or more) of the basedata values (e.g. endpoint values) to be used.

In an embodiment a combination of these techniques is used, and/ordifferent predefined data value generation methods use different onesof, or different combinations of, these techniques.

Thus in an embodiment, the encoded texture data block includes dataindicating how to generate a set or sets of base data values to be usedwhich data indicates that the generated set of base data values is tocomprise a data value or values generated directly from a set of integervalues encoded in the block, and/or should comprise a data value orvalues derived or interpolated from a set of integer values encoded inthe block, or a combination of such data values.

In an embodiment, the data indicating how to generate the set of basedata values to be used indicates which one of a plurality of selected,in an embodiment predetermined, base data value set generationtechniques or methods is to be used.

In an embodiment, the set of texture data elements that is encoded in agiven texture data block is divided into plural “sub-sets” or“partitions” of texture data elements. In other words, the set oftexture elements encoded as a single block of texture data is in anembodiment sub-divided within the texture data block into pluralseparate texture data “sub-blocks”, or “partitions”. In an embodiment 2,3 or 4 partitions can be used.

Where partitioning is used, each texture data element partition for thetexture data block in an embodiment has its own base value(s) (e.g.colour endpoint value(s)), and/or set of data indicating how to generatea set of base data values to be used to generate data values for theparticular sub-set of the texture data elements that the partitioncorresponds to (i.e. for the sub-set that the partition corresponds to).

This has the advantage that different partitions can use, in effect,different encoding schemes (and in an embodiment, this is done).

For example, each partition of a texture data block could, and in anembodiment does, have its own colour endpoints or colour endpointsencoding, which can be specified independently of the colour endpointsor colour endpoint encodings for the other texture data elementpartitions in the encoded block. For example, one partition could have afull RGB colour encoding scheme but another partition could use agrayscale encoding scheme, within the same encoded texture data block.

In this case, there may, accordingly, be, for example, a single set ofinteger values to be shared by all the partitions encoded in the texturedata block, or each partition may have its own separate set of integervalues.

Where an encoded texture data block can include plural partitions, thenin that case the encoded texture data block in an embodiment furtherincludes information to indicate which partitioning pattern has beenused for the block. This information can take any suitable and desiredform, such as an index, that indicates to the decoder which one of a setof stored predefined partitioning patterns has been used.

In one embodiment, the partitioning patterns are generated using apartitioning pattern generation function, and the encoded texture datablocks in an embodiment then include information to be used by thedecoder to configure the partitioning pattern generation function so asto allow the decoder to generate the particular partitioning patternthat has been used. This information in an embodiment comprises apartitioning pattern generation function index or seed, and the numberof partitions, that were used as inputs to the partitioning patterngeneration function at the encoding stage for generating thepartitioning pattern that was used. (The decoder in an embodiment thenuses this information, together with the position of the texture dataelement to be decoded (i.e. whose value is to be determined), as inputsto the partitioning pattern generation function, to determine whichpartition of the encoded texture data block, the texture data element inquestion belongs to. Once this has been done, the decoder can then,e.g., and in an embodiment, determine the base data values (e.g.endpoint colours) to be used for the partition that the texture dataelement has been determined to belong to, and then use those data valuesto determine the data value (e.g. colour value) to be used for thetexture data element itself.)

As discussed above, an encoded texture data block in an embodimentincludes data indicating how to use respective set(s) of base datavalues (e.g. endpoint colours), or generated set(s) of base data values,for a block to generate data values for the texture data elements of theblock.

The data that is included in the encoded texture data block forindicating how to use the set of base data values (e.g. colour endpointvalues) to generate the data values for the individual texture dataelements of the block can be any suitable such data. In an embodiment,it comprises index data, giving indexes for some or all of the texturedata elements in question, and that can be used to derive the datavalues for the texture data elements from the base data values.

In an embodiment the indexes are used to interpolate the data value fora given texture data element from the base (e.g. endpoint) data values.In an embodiment the index is used as or to derive a weight to compute aweighted sum of the base data values (e.g. endpoint values) (whichweighted sum is then used as the data value for the texture data elementto which the index relates).

An encoded texture data block in an embodiment includes, and/or allowsto be derived, an index for each individual texture data element thatthe encoded texture data block represents. In some arrangements, this isdone by providing (explicitly) in the encoded texture data block anindex value for each and every texture data element that the encodedtexture data block represents.

In other arrangements, the encoded texture data block does not encode(explicitly include) all of the indexes to be used for the texture dataelements that the encoded block represents (or indeed any of the indexesto be used for the texture data elements of the block), but insteadincludes (encodes) a set of index values (indexes) from which theindexes to be used for the texture data elements that the encoded blockrepresent will be derived in use by the decoder, e.g., and in anembodiment, by interpolation (and in an embodiment by bilinear orsimplex or trilinear interpolation) from the set of index valuesincluded in the encoded block. This further enhances the flexibility ofthe encoding scheme, as it allows the number of indexes that areprovided to be varied (and to not have to correspond to the number oftexture data elements that an encoded block represents). This may beuseful, in particular for larger block sizes.

Thus, in an embodiment, the decoder derives the indexes to use for theencoded block from a set of indexes that are provided in the encodedtexture data block, in an embodiment by interpolating the indexes to usefrom the indexes that are provided. For example, an index to use may becomputed as a weighted sum of 2, 3, or 4 (or more) of the indexesincluded in the encoded block. The interpolation could use, e.g.,bilinear, trilinear or simplex interpolation, e.g. depending uponwhether the block is a 2D or a 3D block. The set of indices that areprovided in the encoded texture data block is in an embodiment a reducedset of indexes, i.e. contains fewer indexes than the number of texturedata elements that the block represents.

Alternatively or additionally, any “missing” indexes could bedetermined, e.g., using a look-up table or tables, or predeterminedindex “infill” patterns for use to derive the indexes to use could bedefined, e.g., for each different combination of block size and numberof indexes that could be provided explicitly in the encoded texture datablock. These index “infill” patterns could specify, e.g., the weight orweights to be used when summing the provided indexes to derive the“missing” indexes.

In one embodiment, a texture data element can have plural indexesassigned to it. In an embodiment one or two index planes can be used.Where two index planes are used (i.e. two indexes rather than one arespecified for the texture data elements), one index is in an embodimentused for three of the colour components (e.g. the red, green and bluecolour components), and the other index for one colour component (e.g.the alpha colour component). Other arrangements would, of course, bepossible.

The index values included in an encoded texture data block are in anembodiment in the form of integer values from a defined (allowable)range of integer values (as discussed above). Thus, a given encodedtexture data block in an embodiment includes a set of index values takenfrom a defined from a range of integer values (i.e. each index valueincluded in the block will be an integer from the defined (permitted)index value range). The integers representing the index values from thisrange can then be, and are in an embodiment, encoded in the encodedtexture data block in the manner of the technology described herein.

In this arrangement, the permitted (defined) index value range willaccordingly determine the effective resolution available for the indexvalues encoded in the block. Using a larger range of index values willgive a higher resolution, thus, potentially, facilitating greateraccuracy in terms of being able to define a given index value (but atthe cost of needing more bits to encode each index value), whereas usinga more restricted range of index values will reduce the encoded blockcapacity that is required for the index values (but reduce the potentialaccuracy of the index values).

In an embodiment, the defined range for the index values can bespecified for encoded texture data blocks individually. This would thenallow, e.g., some blocks to use a greater range of index values, andother encoded texture data blocks to use a more restricted range ofindex values. This enhances the flexibility of the system of thetechnology described herein.

As discussed above, in an embodiment a range of different index rangescan be defined and are supported (i.e. the indexes can be encoded across(extend over) different ranges of permitted index values), in anembodiment on a block-by-block basis.

In these arrangements, the indexes are in an embodiment scaled fromwhatever range they are defined for (i.e. extend across), into the range0 . . . 1, before being used to derive the actual data value for atexture data element in the decoding process.

It will be appreciated that where different index ranges, etc., may beused and included in an encoded texture data block, the decoder willneed to know the particular “index” encoding arrangement that has beenused. Thus, in an embodiment, information to indicate this is includedin the encoded texture block, in an embodiment by including in theencoded texture data block information indicating a predetermined “indexmode” to be used (that has been used) for the block. In an embodimentthere are plural predefined index modes, with each index mode beingassociated with a particular index range, etc.

The technology described herein can be used to encode any suitable formof texture data. As discussed above, such data, in its original, raw orunencoded form, is typically arranged in the form of arrays of textureelements or texels, and thus in an embodiment, the technology describedherein is used to encode an array of texture data elements (texels).Such arrays are typically, as is known in the art, 2-dimensional,although it is also possible to use the technology described herein toencode a 3-dimensional array (and, indeed, it is an advantage of thetechnology described herein that it can be used to encode 3-dimensionaltextures in an efficient manner).

The texture to be encoded and the texture data elements can representany suitable texture data. In one embodiment the texture is a texturethat is to be used for graphics processing, e.g., and in an embodiment,when rendering an image and/or frame for display, such as for example animage to be applied to primitives to be rendered, colours (includinggrayscale), luminances, bump-maps, shadow-maps (light-maps), etc., as isknown in the art.

However, the technology described herein can also be used to process andencode (and decode) textures to be used to process, and that represent,other forms of data, e.g. where it is desired to use a graphics texture(and graphics texture processing) to represent and process other formsof data. As is known in the art, textures can be and are used ingraphics processing to represent and process many different kinds ofdata, such as, 3D fog, flow fields, etc. as well as for “traditional”graphics purposes. The technology described herein can equally beapplied to, and extends to, these kinds and uses of textures in, andfor, graphics processing systems. Thus the texture of the technologydescribed herein may represent other forms of data that are to beprocessed as a texture in a graphics processing system, if desired.

In an embodiment, the texture data elements each represent a colourvalue for a texture element, but this is not essential. In an embodimentthe texture data elements represent: low dynamic range (LDR) texturedata with 1, 2, 3 or 4 components per texel (luminance, luminance-alpha,RGB and RGB-alpha, respectively) or high dynamic range (HDR) texturedata with 1, 3 or 4 components per texel.

As will be appreciated by those skilled in the art, the actual datavalues accorded to the texture elements, both in their original,unencoded raw form, and in their encoded form (or at least when theencoded data has been decoded to reproduce a representation of theoriginal data) will depend on the form of “texture” that the textureelements are intended to define or represent.

Thus, for example, where the texture elements define colours to be used,the texture data elements in their original, unencoded form may eachcomprise a set of colour values (Red, Green, Blue (RGB), a set of colourand transparency values (Red, Green, Blue, Alpha (RGBa)), or a set ofluminance and chrominance values, and the encoded data, when decoded(reproduced), will generate a corresponding set of colour values.

In the case of shadow (light)-maps, for example, the texture dataelements, will each comprise or represent a set of data valuesindicating, e.g., whether the texture element is in light or in shadow,and the amount (and/or colour) of the light or shadow for that textureelement. Similarly, for a normal-map (bump-map), the data for eachtexture element will be a set of values indicating the direction inwhich light will be reflected at that texture element.

The texture data elements could also, e.g., represent z values (depthvalues), stencil values, luminance values (luminance textures),luminance-alpha-textures, and/or gloss-maps (i.e. whether a surface isshiny at the texture element position or not), etc.

It would be possible, where appropriate for each texture data block toonly encode data necessary to generate some, but not all, of the datavalues necessary to reproduce the original data, with the remaining datavalues being derived (e.g. therefrom) as part of the data reproductionprocess. Thus, in one embodiment, the encoded texture data block encodesdata representative of some of the original texture data, with datarepresentative of other parts of the original data being derived fromthe encoded data during the decoding process.

For example, in the case of normal-maps, it would be possible for theencoded texture data to, e.g., only encode two of the normal directioncomponents (e.g. dx and dy), with the third component (dz) being derivedfrom these two values when the data is decoded (since it is known thatthe sum of the squares of the components must be 1 (as they define anormal vector of length 1): 1=dx²+dy²+dz²).

It should be noted here that references herein to “colours” or“luminances”, etc., accordingly refer to, as will be appreciated bythose skilled in the art, a set of data values that allow the colour orluminance, etc., in question to be reproduced, e.g., on a display. Thesedata values may comprise any suitable set of colour or luminance, etc.,data, such as a set of RGBa values as discussed above. Equally,references to generating a set of data values for an encoded texturedata block, and to data values for individual texture elements,encompass data values that each comprise or include a combination ofvalues, such as a set of colour values (RGB or RGBa), as appropriate.

The set or array of texture data elements that is encoded in accordancewith the technology described herein can be any suitable or desired suchset. For example, the encoded texture data block could, if desired,represent the entire texture map to be reproduced.

However, in an embodiment, each texture data block encodes a smallerportion (or block) of the texture map in question, as in traditionalblock encoding techniques. In such a case, the texture data block willencode and represent a selected set or array of the original texturedata elements. Each texture data block in an embodiment encodes a 4×4,5×5, 6×6, 8×8, 10×10, 12×12, 3×3×3, 4×4×4, 5×5×5 or 6×6×6 array oftexels. (It is an advantage of the technology described herein that itcan support many different block sizes.)

It will be appreciated that in such arrangements, a plurality of suchindividual texture data blocks will be needed to encode the overall setof original texture data (texture data elements), e.g. make-up theentire texture map. Thus, in an embodiment, the technology describedherein includes subdividing a set of texture data elements (e.g. for atexture map) into a plurality of sub-sets of texture data elements, andthen encoding each sub-set of texture data elements as a texture datablock in accordance with the technology described herein. In anembodiment the texture map being encoded is divided into blocks of equalsize, i.e. each sub-set of the texture map represents the same number(and, e.g., array) of texture data elements. This can, e.g., simplifythe task of finding which block a given texture data element lies in. Inan embodiment each encoded texture data block has the same size, i.e. afixed rate encoding scheme is used for the texture map in question. Thisfacilitates the encoding and decoding processes, as is known in the art.Thus, for example, a texture map could be divided into a plurality of4×4 texel arrays, with each such array being encoded as a separatetexture data block.

Where plural texture data blocks are used to encode a larger texture map(or set or array of texture elements) (or part thereof), the actualsubdividing of the array into smaller blocks, and the order of encodinginto texture blocks can be selected as desired. In an embodiment theblocks (sub-sets of data) are encoded in raster order, although otherarrangements, such as the use of Morton order, would, of course, bepossible.

The encoding process of the technology described herein (i.e. to producea set of encoded data blocks of the form discussed above) can be carriedout in any suitable manner on or using the original texture data that isto be encoded. For example, as in known prior art processes, theoriginal data (e.g. texture map) could be, and in an embodiment is,divided into blocks, and then each block encoded using some or all ofthe various different encoding possibilities that are available (i.e.that, in effect, an encoded texture data block can represent). This willprovide a set of possible encoded blocks that can then be compared withthe original data, so as to determine, e.g., which encoded version ofthe block gives the least error (on reproduction) when compared to theoriginal data (which encoding arrangement can then be selected as theone to use for that original texture data block when it is encoded).

This will then be repeated for each different block that the originaldata (e.g. texture map) has been divided into, to produce a stream orset of encoded texture data blocks representing, and corresponding to,the original set of data (e.g. texture map). This set of encoded texturedata blocks can then be stored, e.g. on a portable storage device suchas a DVD, for later use when it is desired to apply the texture to animage to be rendered. In an embodiment a texture is encoded as a set ofmipmaps, with each mipmap being generated in the manner of thetechnology described herein.

Each block that the original data (e.g. texture map) is divided into isin an embodiment the same size and configuration. The block size that isbeing used is in an embodiment provided to the decoder. The block sizeis in an embodiment not included in the encoded texture data blocksthemselves, but is in an embodiment provided to the decoder separately.For example, the block size could be implicitly indicated by anotherparameter that is provided to the decoder, such as, and in anembodiment, the image type, or included in (and indicated in) a (global)data header that is associated with (attached to) the set of encodedtexture data blocks.

It can be seen from the above that in an embodiment of the technologydescribed herein, an encoded texture data block will include dataindicating how to generate a set of base data values to be used togenerate data values for a set of the texture data elements that theblock represents (which data is in an embodiment in the form of an indexor other indication which indicates which of a plurality of predefineddata value set generation techniques or methods is to be used); a set ofinteger values to be used to generate the set of base data values to beused to generate data values for a set of the texture data elements thatthe block represents; a set of index values indicating how to use thegenerated set of base data values to generate data values for textureelements of the set of texture data elements that the generated set ofbase data values to be used for (which index values are in an embodimentused to interpolate the data value for a given texture data element fromthe generated base data values); and information to allow the indexingscheme for the block to be determined (which information is in anembodiment in the form of an index or flag indicating a predetermined“index mode” to be used for the block). The set of integer values to beused to generate the set of base data values, and/or the set of indexvalues, are in an embodiment encoded in the encoded texture data blockin the manner of the technology described herein.

Thus, an embodiment of the technology described herein comprises amethod of encoding a set of texture data elements to be used in agraphics processing system, comprising:

encoding the set of texture data elements as a block of texture datarepresenting the texture data elements; and

including in the texture data block:

data indicating how to generate a set of base data values to be used togenerate data values for a set of the texture data elements that theblock represents;

data indicating a set of integer values to be used to generate the setof base data values to be used to generate data values for a set of thetexture data elements that the block represents;

data indicating a set of index values indicating how to use thegenerated set of base data values to generate data values for texturedata elements of the set of texture data elements that the generated setof base data values is to be used for; and

data indicating the indexing scheme that has been used for the block;

wherein at least the set of integer values to be used to generate theset of base data values, and/or the set of index values, compriseinteger values that are encoded in the encoded texture data block in themanner of the technology described herein.

An embodiment of the technology described herein comprises an apparatusfor encoding a set of texture data elements to be used in a graphicsprocessing system, comprising:

processing circuitry for encoding the set of texture data elements as ablock of texture data representing the texture data elements; and

processing circuitry for including in the texture data block:

data indicating how to generate a set of base data values to be used togenerate data values for a set of the texture data elements that theblock represents;

data indicating a set of integer values to be used to generate the setof base data values to be used to generate data values for a set of thetexture data elements that the block represents;

data indicating a set of index values indicating how to use thegenerated set of base data values to generate data values for texturedata elements of the set of texture data elements that the generated setof base data values is to be used for; and

data indicating the indexing scheme that has been used for the block;

wherein at least the set of integer values to be used to generate theset of base data values, and/or the set of index values, compriseinteger values that are encoded in the encoded texture data block in themanner of the technology described herein.

An embodiment of the technology described herein comprises a block oftexture data representing a set of texture data elements to be used in agraphics processing system, comprising:

data indicating how to generate a set of base data values to be used togenerate data values for a set of the texture data elements that theblock represents;

data indicating a set of integer values to be used to generate the setof base data values to be used to generate data values for a set of thetexture data elements that the block represents;

data indicating a set of index values indicating how to use thegenerated set of base data values to generate data values for texturedata elements of the set of texture data elements that the generated setof base data values is to be used for; and

data indicating the indexing scheme that has been used for the block;

wherein at least the set of integer values to be used to generate theset of base data values, and/or the set of index values, compriseinteger values that are encoded in the encoded texture data block in themanner of the technology described herein.

An embodiment of the technology described herein comprises a method ofencoding a set of texture data elements to be used in a graphicsprocessing system, comprising:

encoding the set of texture data elements as a block of texture datarepresenting the texture data elements; wherein

the encoding process comprises:

determining a set of integer values to be used when generating datavalues for a set of the texture data elements that the block represents;and

encoding the set of integer values in the encoded texture data block inthe manner of the technology described herein.

An embodiment of the technology described herein comprises an apparatusfor encoding a set of texture data elements to be used in a graphicsprocessing system, comprising:

processing circuitry for encoding the set of texture data elements as ablock of texture data representing the texture data elements;

processing circuitry for determining a set of integer values to be usedwhen generating data values for a set of the texture data elements thatthe block represents; and

processing circuitry for encoding the set of integer values in theencoded texture data block in the manner of the technology describedherein.

An embodiment of the technology described herein comprises a block oftexture data representing a set of texture data elements to be used in agraphics processing system, comprising:

data indicating a set of integer values to be used when generating datavalues for a set of the texture data elements that the block represents;

wherein the set of integer values are encoded in the encoded texturedata block in the manner of the technology described herein.

As will be appreciated by those skilled in the art, these embodiments ofthe technology described herein can, and in an embodiment do, includeany one or more or all of the preferred and optional features of thetechnology described herein described herein, as appropriate.

Where the texture data elements that the encoded texture data block havebeen partitioned (as discussed above), the encoded texture data block inan embodiment further includes, as discussed above, information to allowthe partition that a particular texture data element belongs to, to bedetermined.

Where partitioning has been used, the encoded texture data block mayalso, and in an embodiment does also, include, as discussed above, aseparate set of data indicating how to generate a set of base datavalues to be used to generate the data values for a set of the texturedata elements that the block represents for each partition that thetexture data elements of the block have been divided into, in anembodiment together with appropriate integer values to be used for eachrespective partition.

The information to be included in an encoded texture data block can beincluded in the encoded texture data block in any desired order andarrangement, but in a embodiment a particular (and in an embodimentcommon) format and organisation is used for each encoded texture datablock.

Where the encoded block is to include information of the form discussedabove, then the format in an embodiment comprises including the indexmode and data indicating how to generate a set of base data values to beused to generate data values for a set of texture data elements that theblock represents in a particular portion (e.g. at the beginning) of theblock, together with, where necessary, partitioning information (wherepartitioning is being used).

The remaining space of the block is in an embodiment then used to holdthe index data, the integer value data for use to generate the set(s) ofbase data values to be used to generate base data values for the texturedata elements that the block represents, and any needed further dataindicating how to generate a set of base data values to be used togenerate data values for a set of the texture data elements that theblock represents where there is insufficient space in the portion of theblock predefined for that data (e.g. because partitioning is beingused).

In an embodiment, the index data is stored in the remaining space byadding the index data from the top down, and the integer values and anyfurther data indicating how to generate a set of base data values to beused is stored in the remaining space from the bottom up (orvice-versa).

The above primarily describes the encoding of integer values in thetechnology described herein. As will be appreciated by those skilled inthe art, the technology described herein also extends to the reverse,decoding, process, i.e. in which an encoded texture data block is usedto produce one or more integer values for use when decoding an encodedtexture data block to produce one or more or an array of texture dataelements for use (and those integer values are then used to derive thevalue of a texture data element that the encoded block of texture datarepresents).

The decoding process in an embodiment first comprises determining whichencoded texture data block in the set of encoded texture data blocksrepresenting the texture map to be used represents (contains) thetexture data element whose value is required (i.e. that is to bedecoded). This may be done, e.g., and in an embodiment, based on theposition of the texture data element (and, e.g., knowledge of the blocksize and size of the texture). The identified encoded texture data blockcan then be used to determine the value to be used for the texture dataelement in question.

The so-generated, decoded texel values can then be applied, as is knownin the art, to sampling positions and/or fragments that are beingrendered to generate rendered data for those sampling positions and/orfragments, which rendered data is then, e.g. written to a frame bufferfor a display to display the “textured” sampling positions and/orfragments.

Thus, the technology described herein also extends to a decoder and adecoding apparatus configured to decode a texture that has been encodedin the manner of the technology described herein.

Thus, an embodiment of the technology described herein comprises amethod of decoding a texture data block that encodes a set of texturedata elements to be used in a graphics processing system (and in anembodiment in which integer values encoded in the encoded texture datablock are encoded using base-n values, where n is greater than two), themethod comprising:

determining from the encoded texture data block a base-n value, where nis greater than two, and using the base-n value to determine an integervalue encoded in the encoded texture data block.

An embodiment of the technology described herein comprises an apparatusfor decoding a texture data block that encodes a set of texture dataelements to be used in a graphics processing system (and in anembodiment in which integer values encoded in the encoded texture datablock are encoded using base-n values, where n is greater than two), theapparatus comprising:

processing circuitry for using a base-n value, where n is greater thantwo, included in the encoded texture data block to determine an integervalue encoded in the encoded texture data block.

The decoding process can, and indeed in an embodiment does, include, asappropriate, one or more or all of the various preferred and optionalfeatures of the technology described herein discussed herein.

Thus, for example, the determined integer value is in an embodiment thenused to determine the value of a texture data element that the encodedblock of texture data represents. The so-generated, decoded texel valuesare in an embodiment applied to, or used for, as is known in the art,sampling positions and/or fragments that are being rendered to generaterendered data for those sampling positions and/or fragments, whichrendered data is then in an embodiment written to a frame buffer for adisplay to display the “textured” sampling positions and/or fragments.

Similarly, the integer value in an embodiment represents an index value,a base data value, or a value to be used to derive a base data value,for the encoded texture data block.

In an embodiment the decoding process and decoder include steps of orprocessing circuitry for determining from a bit representation includedin the encoded texture data block, plural base-n values, where n isgreater than two, that are encoded in the encoded texture data block,and then using the determined base-n values to determine integer valuesencoded in the encoded texture data block.

Thus, another embodiment of the technology described herein comprises amethod of decoding a texture data block that encodes a set of texturedata elements to be used in a graphics processing system (and in anembodiment in which base-n values, where n is greater than two, are usedto represent integer values encoded in the encoded texture data), themethod comprising:

determining from a bit representation included in the encoded texturedata block that represents plural base-n values, where n is greater thantwo, a base-n value (n>2) that is encoded in the encoded texture datablock, and using the determined base-n value to determine an integervalue encoded in the encoded texture data block.

An embodiment of the technology described herein comprises an apparatusfor decoding a texture data block that encodes a set of texture dataelements to be used in a graphics processing system (and in anembodiment in which base-n values, where n is greater than two, are usedto represent integer values encoded in the encoded texture data), theapparatus comprising:

processing circuitry for determining from a bit representation includedin the encoded texture data block that represents plural base-n values,where n is greater than two, a base-n value (n>2) that is encoded in theencoded texture data block; and

processing circuitry for using the determined base-n value to determinean integer value encoded in the encoded texture data block.

Similarly, in an embodiment the decoding process and decoder includesteps of or processing circuitry for determining from an interleavedsequence of bits in the encoded texture data block, bits of a bitrepresentation representing the values of base-n values (n>2) and bitsrepresenting the values of base-2 values encoded in the encoded texturedata block; determining the base-n values (n>2) and the base-2 valuesfrom the determined bits; and determining a set of integer valuesencoded in the encoded texture data block using a combination of thedetermined base-n values (n>2) and base-2 values.

An embodiment of the technology described herein comprises a method ofdecoding a texture data block that encodes a set of texture dataelements to be used in a graphics processing system, the methodcomprising:

determining from an interleaved sequence of bits in the encoded texturedata block, bits of a bit representation representing the values ofbase-n values (n>2) and bits representing the values of base-2 valuesencoded in the encoded texture data block;

determining one or more of the base-n values (n>2) and of the base-2values from the determined bits; and

determining one or more integer values encoded in the encoded texturedata block using a combination of the determined base-n values (n>2) andbase-2 values.

An embodiment of the technology described herein comprises an apparatusfor decoding a texture data block that encodes a set of texture dataelements to be used in a graphics processing system, the apparatuscomprising:

processing circuitry for determining from an interleaved sequence ofbits in the encoded texture data block, bits of a bit representationrepresenting the values of base-n values (n>2) and bits representing thevalues of base-2 values encoded in the encoded texture data block;

processing circuitry for determining one or more of the base-n values(n>2) and of the base-2 values from the determined bits; and

processing circuitry for determining one or more integer values encodedin the encoded texture data block using a combination of the determinedbase-n values (n>2) and base-2 values.

As will be appreciated by those skilled in the art, all of thesedecoding in an embodiments of the technology described herein can, andin an embodiment do, include any one or more or all of the preferred andoptional features of the technology described herein described herein,as appropriate. Thus, for example, the base-n values (n>2) in anembodiment comprise trits or quints.

As discussed above, the decoding process in an embodiment comprisesdetermining a range that has been used for a set of integer valuesencoded in the encoded texture data block. Similarly, where a set ofinteger values encoded in the encoded texture data block (such as, andin an embodiment, a set of integer values to be used to generate a setof base data values (e.g. endpoint colours) to be used for the encodedtexture data block) is encoded as a restricted, defined range of integervalues, then the decoder in an embodiment converts those integer valuesto appropriate data values to be used (i.e. unquantises the integervalues). For example, each texture data element's colour value may beable to fall within the range 0 . . . 255 (for each component), and so agiven encoded integer value for use as a colour value will need to beconverted from its position within the range that was used for theencoding, to the corresponding position within the range that the colourvalues can take, such as 0 . . . 255, when it is to be used to determinethe colour value for a texture data element.

Thus, in an embodiment, the decoding process comprises converting, andthe decoder converts, a given encoded integer value from its positionwithin a range that was used for the encoding, to a correspondingposition within a larger permitted range that the values that theinteger value corresponds to or is to be used for can take.

It is again believed that such arrangements may be new and advantageousin their own right.

Thus, an embodiment of the technology described herein comprises amethod of decoding a block of encoded texture data representing a set oftexture data elements to be used in a graphics processing system,comprising:

using at least one integer value from a set of integer values encoded inthe encoded texture data block to generate a data value for a texturedata element that the block represents; wherein:

the integer values in the set of integer values are constrained to befrom a restricted range of permitted integer values; and

the method further comprises:

converting the at least one encoded integer value from its positionwithin the range that was used for the encoding, to a correspondingposition within a larger permitted range of integer values, before usingthe at least one integer value to generate a data value for a texturedata element that the block represents.

An embodiment of the technology described herein comprises an apparatusfor decoding a block of encoded texture data representing a set oftexture data elements to be used in a graphics processing system,comprising:

processing circuitry for using at least one integer value from a set ofinteger values encoded in the encoded texture data block to generate adata value for a texture data element that the block represents;wherein:

the integer values in the set of integer values are constrained to befrom a restricted range of permitted integer values; and

the apparatus further comprises:

processing circuitry for converting the at least one encoded integervalue from its position within the range that was used for the encoding,to a corresponding position within a larger permitted range of integervalues before using the at least one integer value to generate a datavalue for a texture data element that the block represents.

As will be appreciated by those skilled in the art, these embodiments ofthe technology described herein can, and in an embodiment do, includeany one or more or all of the preferred and optional features of thetechnology described herein described herein, as appropriate.

The conversion (unquantisation) of the integer values can be done in anysuitable and desired manner. For example, for integer values encodedusing bits (base-2-values) only, the conversion (unquantisation) can bedone using bit replication.

Where the encoded integer values are represented and encoded usingbase-n (n>2) values, then in an embodiment the “unquantisation” isperformed using a predefined process, or series, of bit manipulations.This has been found to provide a relatively accurate method for doingthis, and without the need to use any multipliers.

It is again believed that such arrangements may be new and advantageousin their own right.

Thus, an embodiment of the technology described herein comprises amethod of decoding a block of encoded texture data representing a set oftexture data elements to be used in a graphics processing system,comprising:

using at least one integer value from a set of integer values encoded inthe encoded texture data block to generate a data value for a texturedata element that the block represents; wherein:

the integer values in the set of integer values are constrained to befrom a restricted range of permitted integer values;

the integer values are represented in the encoded texture data blockusing base-n values, where n is greater than two; and

the values of the base-n values (n>2) for the integer values arerepresented using bit representations; and

the method further comprises:

converting the at least one encoded integer value from its positionwithin the range that was used for the encoding, to a correspondingposition within a larger permitted range of integer values, before usingthe at least one integer value to generate a data value for a texturedata element that the block represents, using a series of bitmanipulations on the bit representation for the integer value.

An embodiment of the technology described herein comprises an apparatusfor decoding a block of encoded texture data representing a set oftexture data elements to be used in a graphics processing system,comprising:

processing circuitry for using at least one integer value from a set ofinteger values encoded in the encoded texture data block to generate adata value for a texture data element that the block represents;wherein:

the integer values in the set of integer values are constrained to befrom a restricted range of permitted integer values;

the integer values are represented in the encoded texture data blockusing base-n values, where n is greater than two; and

the values of the base-n values (n>2) for the integer values arerepresented using bit representations; and

the apparatus further comprises:

processing circuitry for converting the at least one encoded integervalue from its position within the range that was used for the encoding,to a corresponding position within a larger permitted range of integervalues, before using the at least one integer value to generate a datavalue for a texture data element that the block represents, using aseries of bit manipulations on the bit representation for the integervalue.

As will be appreciated by those skilled in the art, these embodiments ofthe technology described herein can, and in an embodiment do, includeany one or more or all of the preferred and optional features of thetechnology described herein described herein, as appropriate.

In an embodiment, the bit manipulations include multiplying the base-n(n>2) values for the integer by a predefined constant value, whichconstant is defined based on the range used when encoding the integervalues. The constant is in an embodiment selected so that the base-n(n>2) value makes a low-order contribution to the final unquantised(converted) result.

In an embodiment, the integer values are, as discussed above, alsoencoded, at least in part, using base-2 values (bits). In this case, inan embodiment a swizzle is also or instead (and in an embodiment also)performed on some or all of the base-2 value bits (if any) for theencoded integer value as part of the “unquantisation” bit manipulations.This swizzle is again in an embodiment defined based on the range usedwhen encoding the integer values, and in an embodiment performs atruncated multiplication, and in an embodiment acts to make the bits(the base-2 value bits) make a high-order contribution to the finalunquantised result.

In an embodiment some further bit manipulations are performed to try toobtain code point symmetry (so that, for example, for an unquantisedrange 0 . . . 255, if there is an integer value that unquantises to X,there will also be a value (where that is possible) that unquantises to255-X). This process in an embodiment comprises adding the results ofthe constant multiplication and swizzle operations, XORing that valuewith the lowest bit of the encoded integer value, prepending the lowestbit to the resultant value, and then discarding the lowest two bits ofthe result. The remaining bits give the unquantisation (conversion)result (e.g. the data (e.g. colour) value to use).

The decoding process in an embodiment further comprises performing anyother “decoding” steps necessary to produce one or more or an array oftexture data elements for use in graphics processing from the encodedtexture data block. Thus, for example, it in an embodiment comprisesusing the (unquantised if necessary) integer value or values as or toderive an index value or values for a texture data element or elementsthat the block represents, and/or as a data value or values to be usedas or to generate a set or sets of base data values (e.g., and in anembodiment endpoint colour values) to be used for a texture data elementor elements that the block represents.

In an embodiment the decoder determines integer values from twodifferent sets of integer values for the encoded texture data block,with one set to be used as or to derive a set of index values for thetexture data elements that the block represents, and one set to be usedas data value(s) to be used as or to generate a set or sets of base datavalues (e.g., and in an embodiment endpoint colour values) to be usedfor the texture data elements that the block represents.

The overall decoding process will essentially be the reverse of theencoding process, and thus comprise, e.g., determining from the encodedtexture data block how to generate a set of base data values (e.g.endpoint colours) to be used for block, generating that set of base datavalues (e.g. endpoint colours) (in an embodiment using integer valuesencoded in the block for that purpose) and then generating the datavalues (e.g. colours) for individual texture data elements accordingly(again, in an embodiment using integer index values encoded in the blockfor that purpose). The so-generated, decoded texel values can then beapplied, as is known in the art, to sampling positions and/or fragmentsthat are being rendered to generate rendered data for those samplingpositions and/or fragments, which rendered data is then, e.g. written toa frame buffer for a display to display the “textured” samplingpositions and/or fragments.

As will be appreciated from the above, in an embodiment, the decodingprocess for an individual texture data element will comprise reading anddecoding data for generating a set of base data values to be used toderive the data value for the texture data element; reading and decodinginteger values to be used for that process and thereby generating theset of base data values (e.g., and in an embodiment endpoint colourvalues) to be used to determine the data value for the texture dataelement in question; determining an index value for the texture dataelement, using a defined index mode; and interpolating between thegenerated data values (e.g. endpoint colours) using the index value.This then gives the final decoded texture data element data value (e.g.colour).

The decoding process may be repeated for each texture data element ofthe block whose value is required (and for texture data elements inother encoded blocks).

In an embodiment, the decoder (decoding apparatus) is implemented in thedevice that is to use the encoded textures, such as a graphicsprocessor. The decoder is in an embodiment implemented as a dedicatedhardware element that is configured to carry out the decoding process.

Although the technology described herein has been described above withparticular reference to the encoding (and decoding) of texture data ingraphics processing systems, as will be appreciated by those skilled inthe art, it would equally be applicable to the encoding and decoding ofdata in other forms of data processing system, and in particular to datathat is in the form of (or can be expressed in the form of) arrays orblocks of (similar) data elements (e.g. data elements whose valuesfollow a common or similar format and/or that will be or can be used fora similar or the same purpose or that represent similar information). Itis in particular applicable to the encoding of such data where randomaccess to the data is desired.

Such data could comprise, for example, vector-fields used forsimulations, data representing flow-directions for fluids or bouncedirections for impacts, etc. (which data would, for example, beanalogous to the normal-maps (bump-maps) discussed above in relation tographics data), heat-maps, or sound data, etc.

In such arrangements, the data can be encoded and decoded in ananalogous manner to the texture data as described herein.

An embodiment of the technology described herein comprises a method ofrepresenting integer values to be encoded in an encoded data block thatrepresents a set of data elements, the method comprising:

representing an integer value to be encoded in the encoded data blockusing a base-n value, where n is greater than two.

An embodiment of the technology described herein comprises an apparatusfor representing integer values to be encoded in an encoded data blockthat represents a set of data elements, the apparatus comprising:

processing circuitry for representing an integer value to be encoded inthe encoded data block using a base-n value, where n is greater thantwo.

An embodiment of the technology described herein comprises a block ofdata representing a set of data elements, wherein:

one or more integer values encoded in the data block are representedusing a base-n value, where n is greater than two.

An embodiment of the technology described herein comprises a method ofrepresenting base-n values, where n is greater than two, to be used torepresent integer values to be encoded in an encoded data block thatrepresents a set of data elements, the method comprising:

using a predefined bit representation to represent collectively pluralbase-n values, where n is greater than two.

An embodiment of the technology described herein comprises an apparatusfor representing base-n values, where n is greater than two, to be usedto represent integer values to be encoded in an encoded data block thatrepresents a set of data elements, the apparatus comprising:

processing circuitry for using a predefined bit representation torepresent collectively plural base-n values, where n is greater thantwo.

An embodiment of the technology described herein comprises a block ofdata representing a set of data elements, wherein:

plural integer values encoded in the data block are represented usingbase-n values, where n is greater than two; and

the block includes:

a predefined bit representation that represents collectively a pluralityof the base n-values.

An embodiment of the technology described herein comprises a method ofencoding integer values to be encoded in an encoded data block thatrepresents a set of data elements, the method comprising:

representing a set of integer values to be encoded in the encoded datablock using a combination of base-n values, where n is greater than two,and base-2 values;

representing the values of the base-n values (n>2) for the set ofinteger values using bit representations, and representing the values ofthe base-2 values for the set of integer values using bits; and

interleaving the bits of the bit representations representing the valuesof the base-n values (n>2) with the bits representing the values of thebase-2 values in the encoded data block.

An embodiment of the technology described herein comprises an apparatusfor encoding integer values to be encoded in an encoded data block thatrepresents a set of data elements, the apparatus comprising:

processing circuitry for representing a set of integer values to beencoded in the encoded data block using a combination of base-n values,where n is greater than two, and base-2 values;

processing circuitry for representing the values of the base-n values(n>2) for the set of integer values using bit representations, andrepresenting the values of the base-2 values for the set of integervalues using bits; and

processing circuitry for interleaving the bits of the bitrepresentations representing the values of the base-n values (n>2) withthe bits representing the values of the base-2 values in the encodeddata block.

An embodiment of the technology described herein comprises a block ofdata representing a set of data elements, wherein:

the block of data:

represents a set of integer values encoded in the encoded data blockusing a combination of base-n values, where n is greater than two, andbase-2 values; and

represents the values of the base-n values (n>2) for the set of integervalues using bit representations, and represents the values of thebase-2 values for the set of integer values using bits; and wherein:

the bits of the bit representations representing the values of thebase-n values (n>2) are interleaved with the bits representing thevalues of the base-2 values in the encoded data block.

An embodiment of the technology described herein comprises a method ofencoding a set of data elements, comprising:

encoding the set of data elements as a block of data representing thedata elements; and

including in the data block:

a set of integer values (in an embodiment for using to generate a set ofdata values to be used to generate data values for a set of the dataelements that the block represents); wherein:

the integer values are constrained to be from a restricted range ofpermitted integer values; and

the method further comprises:

determining the range to be used for the set of integer values based onthe number of integer values in the set and the space available in theencoded data block for encoding the set of integer values.

An embodiment of the technology described herein comprises an apparatusfor encoding a set of data elements, comprising:

processing circuitry for encoding the set of data elements as a block ofdata representing the data elements; and

processing circuitry for including in the data block:

a set of integer values (in an embodiment for using to generate a set ofdata values to be used to generate data values for a set of the dataelements that the block represents); wherein:

the integer values are constrained to be from a restricted range ofpermitted integer values; and

the apparatus further comprises:

processing circuitry for determining the range to be used for the set ofinteger values based on the number of integer values in the set, and thespace available in the encoded texture data block for encoding the setof integer values.

An embodiment of the technology described herein comprises a method ofdecoding a block of encoded data representing a set of data elements,comprising:

using a set of integer values included in the encoded data block whendecoding the block (and in an embodiment to generate data values for aset of the data elements that the block represents); wherein:

the integer values are constrained to be from a restricted range ofpermitted integer values; and

the method further comprises:

determining the range that has been used for the set of integer valuesbased on the number of integer values in the set, and the spaceavailable in the encoded data block for encoding the set of integervalues.

An embodiment of the technology described herein comprises an apparatusfor decoding a block of encoded data representing a set of dataelements, comprising:

processing circuitry for using a set of integer values included in theencoded data block when decoding the block (and in an embodiment togenerate data values for a set of the data elements that the blockrepresents); wherein:

the integer values are constrained to be from a restricted range ofpermitted integer values; and

the apparatus further comprises:

processing circuitry for determining the range that has been used forthe set of integer values based on the number of integer values in theset, and the available space in the encoded data block for encoding theset of integer values.

An embodiment of the technology described herein comprises a method ofencoding a set of data elements, comprising:

encoding the set of data elements as a block of data representing thedata elements; and

including in the data block:

data indicating how to generate a set of base data values to be used togenerate data values for a set of the data elements that the blockrepresents;

data indicating a set of integer values to be used to generate the setof base data values to be used to generate data values for a set of thedata elements that the block represents;

data indicating a set of index values indicating how to use thegenerated set of base data values to generate data values for dataelements of the set of data elements that the generated set of base datavalues is to be used for; and

data indicating the indexing scheme that has been used for the block;

wherein at least the set of integer values to be used to generate theset of base data values, and/or the set of index values, compriseinteger values that are encoded in the encoded data block in the mannerof the technology described herein.

An embodiment of the technology described herein comprises an apparatusfor encoding a set of data elements, comprising:

processing circuitry for encoding the set of data elements as a block ofdata representing the data elements; and

processing circuitry for including in the data block:

data indicating how to generate a set of base data values to be used togenerate data values for a set of the data elements that the blockrepresents;

data indicating a set of integer values to be used to generate the setof base data values to be used to generate data values for a set of thedata elements that the block represents;

data indicating a set of index values indicating how to use thegenerated set of base data values to generate data values for dataelements of the set of data elements that the generated set of base datavalues is to be used for; and

data indicating the indexing scheme that has been used for the block;

wherein at least the set of integer values to be used to generate theset of base data values, and/or the set of index values, compriseinteger values that are encoded in the encoded data block in the mannerof the technology described herein.

An embodiment of the technology described herein comprises a block ofdata representing a set of data elements, comprising:

data indicating how to generate a set of base data values to be used togenerate data values for a set of the data elements that the blockrepresents;

data indicating a set of integer values to be used to generate the setof base data values to be used to generate data values for a set of thedata elements that the block represents;

data indicating a set of index values indicating how to use thegenerated set of base data values to generate data values for dataelements of the set of data elements that the generated set of base datavalues is to be used for; and

data indicating the indexing scheme that has been used for the block;

wherein at least the set of integer values to be used to generate theset of base data values, and/or the set of index values, compriseinteger values that are encoded in the encoded data block in the mannerof the technology described herein.

An embodiment of the technology described herein comprises a method ofencoding a set of data elements, comprising:

encoding the set of data elements as a block of data representing thedata elements; wherein

the encoding process comprises:

determining a set of integer values to be used when generating datavalues for a set of the data elements that the block represents; and

encoding the set of integer values in the encoded data block in themanner of the technology described herein.

An embodiment of the technology described herein comprises an apparatusfor encoding a set of data elements, comprising:

processing circuitry for encoding the set of data elements as a block ofdata representing the data elements;

processing circuitry for determining a set of integer values to be usedwhen generating data values for a set of the data elements that theblock represents; and

processing circuitry for encoding the set of integer values in theencoded data block in the manner of the technology described herein.

An embodiment of the technology described herein comprises a block ofdata representing a set of data elements, comprising:

data indicating a set of integer values to be used when generating datavalues for a set of the data elements that the block represents;

wherein the set of integer values are encoded in the encoded data blockin the manner of the technology described herein.

An embodiment of the technology described herein comprises a method ofdecoding a block of encoded data representing a set of data elements,comprising:

using at least one integer value from a set of integer values encoded inthe encoded data block to generate a data value for a data element thatthe block represents; wherein:

the integer values in the set of integer values are constrained to befrom a restricted range of permitted integer values; and

the method further comprises:

converting the at least one encoded integer value from its positionwithin the range that was used for the encoding, to a correspondingposition within a larger permitted range of integer values, before usingthe at least one integer value to generate a data value for a dataelement that the block represents.

An embodiment of the technology described herein comprises an apparatusfor decoding a block of encoded data representing a set of dataelements, comprising:

processing circuitry for using at least one integer value from a set ofinteger values encoded in the encoded data block to generate a datavalue for a data element that the block represents; wherein:

the integer values in the set of integer values are constrained to befrom a restricted range of permitted integer values; and

the apparatus further comprises:

processing circuitry for converting the at least one encoded integervalue from its position within the range that was used for the encoding,to a corresponding position within a larger permitted range of integervalues before using the at least one integer value to generate a datavalue for a data element that the block represents.

An embodiment of the technology described herein comprises a method ofdecoding a block of encoded data representing a set of data elements,comprising:

using at least one integer value from a set of integer values encoded inthe encoded data block to generate a data value for a data element thatthe block represents; wherein:

the integer values in the set of integer values are constrained to befrom a restricted range of permitted integer values;

the integer values are represented in the encoded data block usingbase-n values, where n is greater than two; and

the values of the base-n values (n>2) for the integer values arerepresented using bit representations; and

the method further comprises:

converting the at least one encoded integer value from its positionwithin the range that was used for the encoding, to a correspondingposition within a larger permitted range of integer values, before usingthe at least one integer value to generate a data value for a dataelement that the block represents, using a series of bit manipulationson the bit representation for the integer value.

An embodiment of the technology described herein comprises an apparatusfor decoding a block of encoded data representing a set of dataelements, comprising:

processing circuitry for using at least one integer value from a set ofinteger values encoded in the encoded data block to generate a datavalue for a data element that the block represents; wherein:

the integer values in the set of integer values are constrained to befrom a restricted range of permitted integer values;

the integer values are represented in the encoded data block usingbase-n values, where n is greater than two; and

the values of the base-n values (n>2) for the integer values arerepresented using bit representations; and

the apparatus further comprises:

processing circuitry for converting the at least one encoded integervalue from its position within the range that was used for the encoding,to a corresponding position within a larger permitted range of integervalues, before using the at least one integer value to generate a datavalue for a data element that the block represents, using a series ofbit manipulations on the bit representation for the integer value.

An embodiment of the technology described herein comprises a method ofdecoding a data block that encodes a set of data elements (and in anembodiment in which base-n values, where n is greater than two, are usedto represent integer values encoded in the encoded data), the methodcomprising:

determining from a bit representation included in the encoded data blockthat represents plural base-n values, where n is greater than two, abase-n (n>2) value that is encoded in the encoded data block, and usingthe determined base-n value to determine an integer value encoded in theencoded texture data block.

An embodiment of the technology described herein comprises an apparatusfor decoding a data block that encodes a set of data elements (and in anembodiment in which base-n values, where n is greater than two, are usedto represent integer values encoded in the encoded data), the apparatuscomprising:

processing circuitry for determining from a bit representation includedin the encoded data block that represents plural base-n values, where nis greater than two, a base-n (n>2) value that is encoded in the encodeddata block; and

processing circuitry for using the determined base-n value to determinean integer value encoded in the encoded texture data block.

An embodiment of the technology described herein comprises a method ofdecoding a data block that encodes a set of data elements (and in anembodiment in which integer values encoded in the encoded data block areencoded using base-n values, where n is greater than two), the methodcomprising:

determining from the encoded data block a base-n value, where n isgreater than two, and using the base-n value to determine an integervalue encoded in the encoded data block.

An embodiment of the technology described herein comprises an apparatusfor decoding a data block that encodes a set of data elements (and in anembodiment in which integer values encoded in the encoded data block areencoded using base-n values, where n is greater than two), the apparatuscomprising:

processing circuitry for using a base-n value, where n is greater thantwo, included in the encoded data block to determine an integer valueencoded in the encoded data block.

An embodiment of the technology described herein comprises a method ofdecoding a data block that encodes a set of data elements, the methodcomprising:

determining from an interleaved sequence of bits in the encoded datablock, bits of a bit representation representing the values of base-nvalues (n>2) and bits representing the values of base-2 values encodedin the encoded data block;

determining one or more of the base-n values and of the base-2 valuesfrom the determined bits; and

determining one or more integer values encoded in the encoded data blockusing a combination of the determined base-n values, where n is greaterthan two, and base-2 values.

An embodiment of the technology described herein comprises an apparatusfor decoding a data block that encodes a set of data elements, theapparatus comprising:

processing circuitry for determining from an interleaved sequence ofbits in the encoded data block, bits of a bit representationrepresenting the values of base-n values (n>2) and bits representing thevalues of base-2 values encoded in the encoded data block;

processing circuitry for determining one or more of the base-n valuesand of the base-2 values from the determined bits; and

processing circuitry for determining one or more integer values encodedin the encoded data block using a combination of the determined base-nvalues, where n is greater than two, and base-2 values.

As will be appreciated by those skilled in the art, all of theseembodiments of the technology described herein can, and in an embodimentdo, include any one or more or all of the preferred and optionalfeatures of the technology described herein described herein, asappropriate.

The methods and apparatus of the technology described herein can beimplemented in any appropriate manner, e.g. in hardware or software, andin (and be included in) any appropriate device or component. In anembodiment they are implemented in a graphics processor, and thus thetechnology described herein also extends to a graphics processorconfigured to use the methods of the technology described herein, orthat includes the apparatus of the technology described herein. In anembodiment, the methods and apparatus of the technology described hereinare implemented in hardware, in an embodiment on a single semi-conductorplatform.

The technology described herein can be implemented in any suitablesystem, such as a suitably configured micro-processor based system. Inan embodiment, the technology described herein is implemented incomputer and/or micro-processor based system.

The various functions of the technology described herein can be carriedout in any desired and suitable manner. For example, the functions ofthe technology described herein can be implemented in hardware orsoftware, as desired. Thus, for example, the various functional elementsand “means” of the technology described herein may comprise a suitableprocessor or processors, controller or controllers, functional units,circuitry, processing logic, microprocessor arrangements, etc., that areoperable to perform the various functions, etc., such as appropriatelydedicated hardware elements and processing circuitry and/or programmablehardware elements and processing circuitry that can be programmed tooperate in the desired manner. The various functional elements, etc.,may be separate to each other or may share circuitry (e.g. be performedby the same processor), as desired.

It should also be noted here that, as will be appreciated by thoseskilled in the art, the various functions, etc., of the technologydescribed herein may be duplicated and/or carried out in parallel on agiven processor.

The technology described herein is applicable to any suitable form orconfiguration of graphics processor and renderer, such as tile-basedgraphics processors, immediate mode renderers, processors having a“pipelined” rendering arrangement, etc.

As will be appreciated from the above, the technology described hereinis particularly, although not exclusively, applicable to graphicsprocessors and processing devices, and accordingly extends to a graphicsprocessor and a graphics processing platform including the apparatus of,or operated in accordance with the method of, any one or more of theembodiments of the technology described herein described herein. Subjectto any hardware necessary to carry out the specific functions discussedabove, such a graphics processor can otherwise include any one or moreor all of the usual functional units, etc., that graphics processorsinclude.

It will also be appreciated by those skilled in the art that all of thedescribed embodiments of the technology described herein can include, asappropriate, any one or more or all of the preferred and optionalfeatures described herein.

The methods in accordance with the technology described herein may beimplemented at least partially using software e.g. computer programs. Itwill thus be seen that when viewed from further embodiments thetechnology described herein provides computer software specificallyadapted to carry out the methods herein described when installed on adata processor, a computer program element comprising computer softwarecode portions for performing the methods herein described when theprogram element is run on a data processor, and a computer programcomprising code adapted to perform all the steps of a method or of themethods herein described when the program is run on a data processingsystem. The data processing system may be a microprocessor, aprogrammable FPGA (Field Programmable Gate Array), etc.

The technology described herein also extends to a computer softwarecarrier comprising such software which when used to operate a graphicsprocessor, renderer or other system comprising a data processor causesin conjunction with said data processor said processor, renderer orsystem to carry out the steps of the methods of the technology describedherein. Such a computer software carrier could be a physical storagemedium such as a ROM chip, CD ROM or disk, or could be a signal such asan electronic signal over wires, an optical signal or a radio signalsuch as to a satellite or the like.

It will further be appreciated that not all steps of the methods of thetechnology described herein need be carried out by computer software andthus from a further broad in an embodiment the technology describedherein provides computer software and such software installed on acomputer software carrier for carrying out at least one of the steps ofthe methods set out herein.

The technology described herein may accordingly suitably be embodied asa computer program product for use with a computer system. Such animplementation may comprise a series of computer readable instructionseither fixed on a tangible, non-transitory medium, such as a computerreadable medium, for example, diskette, CD ROM, ROM, or hard disk, ortransmittable to a computer system, via a modem or other interfacedevice, over either a tangible medium, including but not limited tooptical or analogue communications lines, or intangibly using wirelesstechniques, including but not limited to microwave, infrared or othertransmission techniques. The series of computer readable instructionsembodies all or part of the functionality previously described herein.

Those skilled in the art will appreciate that such computer readableinstructions can be written in a number of programming languages for usewith many computer architectures or operating systems. Further, suchinstructions may be stored using any memory technology, present orfuture, including but not limited to, semiconductor, magnetic, oroptical, or transmitted using any communications technology, present orfuture, including but not limited to optical, infrared, or microwave. Itis contemplated that such a computer program product may be distributedas a removable medium with accompanying printed or electronicdocumentation, for example, shrink wrapped software, pre loaded with acomputer system, for example, on a system ROM or fixed disk, ordistributed from a server or electronic bulletin board over a network,for example, the Internet or World Wide Web.

An embodiment of the technology described herein will now be describedwith reference to the encoding of texture data for use in graphicsprocessing that is in the form of a colour map (i.e. colour data).However, as discussed above, and as will be appreciated by those skilledin the art, the technology described herein is applicable to dataencoding and decoding in general, and so therefore should not beconsidered to be limited to the present example of texture colour dataencoding.

FIG. 1 illustrates the basic encoding process of this embodiment. Asshown in FIG. 1, an original image or array 1 of texture data elements(texels) (a texture “map”) is subdivided into a plurality of 4×4 textureelement blocks 2. (Other block sizes can be used, as will be discussedfurther below.)

In the present embodiment, as shown in FIG. 1, the original image(texture map) 1 is divided into blocks of equal size. This simplifiesthe task of finding which block a given texture data element lies in,and gives a constant data rate.

In this embodiment, each texture element (texel) in the original texturemap data array (image) represents the colour to be used at the positionof the respective texture element, and accordingly has allocated to it adata value comprising a set of colour values (e.g. red, green, blue(RGB), and, optionally alpha (transparency) values. In other words, inthis embodiment, the data values that are encoded and generated, etc.,each correspond to and represent a colour (a set of colour values). Forconvenience, the following description will therefore refer primarily to“colours” but it should be understood that such references indicate adata value comprising a set of colour values that represent the colourin question.

In the present embodiment, rather than storing the array of colour datain its full, original form, each 4×4 texture element block 2 is encodedas a texture data block 5 that has a reduced size as compared to thetexture data in its original, unencoded form. This, in effect,compresses the original texture data, thereby making its storage andprocessing easier and more efficient. In the present embodiment, eachencoded texture data block 5 uses 128 bits. (Other arrangements would,of course, be possible.)

Each encoded, reduced size, texture data block 5 contains, as will bediscussed further below, sufficient and appropriate data to allow datacorresponding to or representing the original, unencoded, data of the4×4 texture element block in question to be reproduced.

For each block 2 of the original image (texture map) 1, a correspondingencoded texture data block 5 is generated. The individual texture datablocks making up the texture map are encoded in the present embodimentin raster order. Other arrangements, such as the use of Morton order,would, of course, be possible.

Thus, in the present embodiment, each encoded texture data filecomprises a sequence of individual texture data blocks encoding thearray of texture data elements (the image data).

The number of texture data blocks in the file will depend on the size ofthe texture map (texture data array) that is being encoded, and, e.g.,whether the data is being stored in mip-map form. If mip-maps are used,then if the first level of texture map resolution is encoded using “n”texture data blocks, there will be “n/4” texture data blocks for themip-map level above, “n/16” blocks for the next mip-map, “n/64” for thenext, “n/256” for the next again, and so on (but no less than one blockfor each level) until the mip-map level with size 1×1 is reached.

(In the present embodiment, the encoded texture data can be and in anembodiment is stored in the form of mip-maps (i.e. where multipleversions of the original texture data array, each having differentlevels of detail (resolution), are stored for use). The mip-maps are inan embodiment stored one after each other in memory, with each mip-maplevel being, as is known in the art, a downscaled (by a factor of 2)representation (image) of the original array (image). The mip-maps arestored in order of descending resolution, i.e. n×n, . . . , 16×16, 8×8,4×4, 2×2, 1×1. The smaller mip-maps (<8×8) are each stored individuallyin a separate encoded data block.)

As will be discussed further below, the present embodiment supportsarrangements in which the texture data elements (texels) in a giventexel block to be encoded are divided into different sub-sets orpartitions within the block. FIG. 2 illustrates this, and shows a 4×4texel block 2 which has been divided into three partitions 10, 11 and12. Thus the texels labelled “a” in FIG. 2 belong to a first partition10, the texels labelled “b” belong to a second partition 11, and thetexels labelled “c” belong to a third partition 12. This block is thenencoded in a compressed form as an encoded texture data block 13, butwith, as will be explained in more detail below, additional informationrelating to the partitioning of the original 4×4 texel block.

The format for encoding (and decoding) a block of texture data elements(texels) that is used in the present embodiment will now be described.As will be discussed further below, the encoding format of the presentembodiment includes sequences of integer values in the encoded texturedata blocks and uses the integer value encoding techniques of thetechnology described herein to encode those sequences of integer values.

Overview

The present embodiment uses a texture compression format designed toprovide lossy texture compression suitable for a wide range of differenttypes of content and a wide range of quality/bitrate tradeoffs. Theformat has the following main features:

-   -   128-bit block size    -   an encoded block is self-contained (any given texel is        completely defined by the contents of a single block)    -   Designed for compression of the following types of texture data:        -   LDR (low dynamic range) texture data with 1, 2, 3 or 4            components per texel (Luminance, Luminance-Alpha, RGB and            RGB-Alpha, respectively)        -   HDR (high dynamic range) texture data with 1, 3 or 4            components per texel    -   Fine-grained per-block adjustable bit-allocation between index        bits and color endpoint bits.    -   2D and 3D variants.    -   Each block represents a rectangular or cuboidal footprint of        texels. The footprint size determines the bit-rate of this        texture format and is global for the texture as a whole.

Supported footprint sizes for 2D variants are:

-   -   4×4 (8 bpp)    -   5×4 (6.40 bpp)    -   5×5 (5.12 bpp)    -   6×5 (4.27 bpp)    -   6×6 (3.56 bpp)    -   8×5 (3.20 bpp)    -   8×6 (2.67 bpp)    -   10×5 (2.56 bpp)    -   10×6 (2.13 bpp)    -   8×8 (2 bpp)    -   10×8 (1.60 bpp)    -   10×10 (1.28 bpp)    -   12×10 (1.07 bpp)    -   12×12 (0.88 bpp)

Supported footprint sizes for 3D variants are:

-   -   3×3×3 (4.74 bpp)    -   4×3×3 (3.56 bpp)    -   4×4×3 (2.67 bpp)    -   4×4×4 (2 bpp)    -   5×4×4 (1.60 bpp)    -   5×5×4 (1.28 bpp)    -   5×5×5 (1.02 bpp)    -   6×5×5 (0.85 bpp)    -   6×6×5 (0.71 bpp)    -   6×6×6 (0.59 bpp)    -   The types of texture data supported (component count, LDR vs        HDR) is not dependent on footprint size; all types are available        at all sizes.    -   Block partitioning, with a partitioning pattern generation        function; each partition has a separate pair of endpoint colors.        The format allows different partitions within a single block to        have different endpoint types. The format supports 1 to 4        partitions per block.    -   Index decimation: The format allows indices to be specified for        only some texels, with an infill procedure used for the        remaining texels; this is especially useful at lower bitrates.    -   Void extents: The format offers an encoding to indicate large        empty regions within the texture.

The ability to use different data rates for different mipmap levels.

Layout of the Block

If partitioning is disabled for the block, then the encoded block hasthe following format:

Bits Usage 10:0  Index Bits Mode 12:11 “00” 16:13 Color Endpoint Mode127:17  Remaining Space

If partitioning is enabled, the encoded block has the following format:

Bits Usage 10:0  Index Bits Mode 12:11 Partition count minus 1 22:13Partition index 28:23 Color Endpoint Mode, initial six bits 127:29 Remaining Space

The “Remaining Space” is used to hold Index Data (from the top down),Color Endpoint Data (from the bottom up) and Color Endpoint Mode bits(if more than 6 bits are needed). The sizes of the Index Data, the ColorEndpoint Data and the Color Endpoint Mode bits are not fixed, but areinstead computed based on Index Bit Mode and the initial six bits ofColor Endpoint Mode.

As a special case, if bits[8:0] of the encoded block are “111111100”,then the block does not encode ordinary compressed content; instead, itencodes a Void-Extent Block.

Partitioning

An encoded block is subdivided into 1, 2, 3 or 4 partitions, with aseparate color endpoint pair for each partition. The number ofpartitions is specified by the “Partition count minus 1” bits.

If 2 or more partitions are used, then the partition index is used toselect one of 1024 partitioning patterns; the set of patterns supporteddepends on the partition count and block size.

The partitioning patterns are produced with a generator function; thisenables a very large set of partitioning patterns for different blocksizes to be implented with a minimal number of gates. The details on howthe generator works in this embodiment are given below.

Index Modes

The “Index Bits Mode” field controls the number of indexes present, aswell as the range used for them. The set of possible combinations dependon the block dimensionality (2D or 3D).

The actual indexes in the block are stored are follows:

-   -   First, they are encoded using the Integer Sequence Encoding        method described below.    -   The resulting bit-sequence is then bit-reversed, and stored from        the top of the block downwards.        Usage of Indexes

The indexes are used to interpolate between two endpoint colors for eachtexel. First, they are scaled from whatever interval (range) they weresupplied in to the range 0 . . . 1; the resulting value is then used asa weight to compute a weighted sum of the two endpoints. Any suitableunquantization procedure for the scaling to the 0 . . . 1 range can beused.

Index Infill

Each texel that the block encodes has a corresponding index to be usedfor that texel. In some of the index modes, one index is supplied forevery texel in the block; in others, the number of indexes is less thanthe number of texels. In the latter case, the indexes that are actuallyto be used for the texels are derived by bilinear (or simplex ortrilinear, for 3D blocks) interpolation from the indexes that aresupplied (encoded) in the block. Thus, when the index count is smallerthan the number of texels in the block, the actual indexes to be usedfor the texels of the block are derived by bilinear (or simplex ortrilinear) interpolation from the index values supplied in the encodedblock, i.e. the index for a texel will be computed as an appropriatelyweighted sum of 2, 3 or 4 (or more) of the indexes supplied (included)in the encoded block.

Thus, in the present embodiment, where an encoded texture data blockincludes fewer indices than the number of texels the block represents,the encoded texture data block will include a set of index valuesrepresenting an array of index values at a given resolution that is lessthan the resolution of the array of texture data elements that the blockrepresents, and then the index values to use for the array of texturedata elements that the block represents are derived in use by bilinear(or simplex or trilinear) interpolation from the array of index valuesthat is encoded (included) in the encoded texture data block. Forexample, an encoded block encoding an 8×8 array of texels may encode(include) only a 5×5 array of index values.

Other arrangements, such as using look-up tables, and/or usingpredefined index infill patterns (which may be derived, e.g. using apredefined infill pattern generation function, or stored explicitly,e.g. in look-up tables), to derive any “missing” texel indexes can alsoor instead be used if desired.

Index Planes

Depending on the Index Bits mode selected, the format may offer 1 or 2index planes. In the case of 2 index planes, two indexes rather thanjust one are supplied for each texel that receives indexes. Of these twoindexes, the first one is used for a weighted sum of three of the colorcomponents; the second is used for a weighted sum of the fourth colorcomponent. If only 1 index plane is present, it applies to all fourcolor components.

If two index planes are used, then a 2-bit bitfield is used to indicatewhich of the color components the second index plane applies to. Thesetwo bits are stored just below the index bits, except in the case whereleftover color endpoint type bits are present; in that case, these twobits are stored just below the leftover color endpoint type bits.

This two-bit bitfield has the following layout:

Value Meaning 0 Red 1 Green 2 Blue 3 Alpha

If index infill is present while two index planes are being used, thenindex infill is performed on each index plane separately.

Index Modes

The Index Mode field specifies the width, height and depth of the gridof indices, what range of values they use, and whether dual index planesare present. Since some these are not represented using powers of two(there are 12 possible index widths, for example), and not allcombinations are allowed, this is not a simple bit packing. However, itcan be unpacked quickly in hardware.

The index ranges are encoded using a 3 bit value R, which is interpretedtogether with a precision bit H, as follows:

R Index Range Trits Quints Bits Low Precision Range (H = 0) 000 Invalid001 Invalid 010 0 . . . 1 1 011 0 . . . 2 1 100 0 . . . 3 2 101 0 . . .4 1 110 0 . . . 5 1 1 111 0 . . . 7 3 High Precision Range (H = 1) 000Invalid 001 Invalid 010 0 . . . 9 1 1 011 0 . . . 11 1 2 100 0 . . . 154 101 0 . . . 19 1 2 110 0 . . . 23 1 3 111 0 . . . 31 5

For 2D blocks, the index mode field is laid out as follows:

Width Height 10 9 8 7 6 5 4 3 2 1 0 N M Notes D H B A R₀ 0 0 R₂ R₁ B + 4A + 2 D H B A R₀ 0 1 R₂ R₁ B + 8 A + 2 D H B A R₀ 1 0 R₂ R₁ A + 2 B + 8D H 0 B A R₀ 1 1 R₂ R₁ A + 2 B + 6 D H 1 B A R₀ 1 1 R₂ R₁ B + 2 A + 2 DH 0 0 A R₀ R₂ R₁ 0 0 12 A + 2 D H 0 1 A R₀ R₂ R₁ 0 0 A + 2 12 D H 1 1 00 R₀ R₂ R₁ 0 0  6 10 D H 1 1 0 1 R₀ R₂ R₁ 0 0 10  6 B 1 0 A R₀ R₂ R₁ 0 0A + 6 B + 6 D = 0, H = 0 x x 1 1 1 1 1 1 1 0 0 — — Void-extent x x 1 1 1x x x x 0 0 — — Reserved x x x x x x x 0 0 0 0 — — Reserved

Note that, due to the encoding of the R field, as described in theprevious page, bits R₂ and R₁ cannot both be zero, which disambiguatesthe first five rows from the rest of the table.

For 3D blocks, the index mode field is laid out as follows:

Width Height Depth 10 9 8 7 6 5 4 3 2 1 0 N M Q Notes D H B A R₀ C R₂ R₁A + 2 B + 2 C + 2 B 0 0 A R₀ R₂ R₁ 0 0 6 B + 2 A + 2 D = 0, H = 0 B 0 1A R₀ R₂ R₁ 0 0 A + 2 6 B + 2 D = 0, H = 0 B 1 0 A R₀ R₂ R₁ 0 0 A + 2 B +2 6 D = 0, H = 0 D H 1 1 0 0 R₀ R₂ R₁ 0 0 6 2 2 D H 1 1 0 1 R₀ R₂ R₁ 0 02 6 2 D H 1 1 1 0 R₀ R₂ R₁ 0 0 2 2 6 x x 1 1 1 1 1 1 1 0 0 — — —Void-extent x x 1 1 1 1 x x x 0 0 — — — Reserved (except for valid voidextent encodings) x x x x x x x 0 0 0 0 — — — Reserved

The D bit is set to indicate dual-plane mode. In this mode, the maximumallowed number of partitions is 3.

The size of the grid in each dimension must be less than or equal to thecorresponding dimension of the block footprint. If the grid size isgreater than the footprint dimension in any axis, then this is anillegal block encoding and all texels will decode to an error color.

The index range specifies how the index values are used to compute theweightings. In all cases, the value 0 will generate an interpolatedvalue with the value of endpoint 1, and the maximum value (according tothe selected range) generates an interpolated value equal to endpoint2's value.

For LDR endpoint values, the interpolation is linear. So if M is themaximum allowed value in the range, the actual interpolated value isequal to (1−index/M)*(endpoint value 1)+(index/M)*(endpoint value 2).The division by M is what scales the input values in the range 0 . . . Minto weighting values in the range 0 . . . 1. The range thereforeselects how many intermediate steps there are between these two values.The more range, the more likely one is able to represent a valueclosely, but the more bits needed to encode it.

In the present embodiment, the index value is first rescaled so that Mis a power of two (in an embodiment 64), so that the costly division byM can be replaced with a relatively cheap multiplication by 64/M, andthen a division by 64.

For HDR endpoint values, the interpolation is a logarithmic function, oran approximation thereof. The endpoint values are encoded as logarithmsto the base 2 of the original endpoint values. So if M is the maximumallowed value in the range, the interpolated value is the logarithm ofthe final decoded values, and is equal to (1−index/M)*(endpoint value1)+(index/M)*(endpoint value 2). The final decoded value is therefore 2to the power of the interpolated value.

In the present embodiment, the HDR endpoint values are stored as valueswith a 12 bit floating point representation, and interpolation occurs ina piecewise-approximate logarithmic manner as follows.

The HDR color components from each endpoint, C0 and C1, are initiallyshifted left 4 bits to become 16-bit integer values and these are firstinterpolated in the same way as LDR, using the rescaled index value i.The resulting 16-bit value C is then decomposed into the top five bits,e, and the bottom 11 bits m, which are then processed and recombinedwith e to form the final value Cf:

C=floor((C0*(64−i)+C1*i+32)/64)

E=(C&0xF800)>>11; m=C&0x7FF;

if (m<512) {mt=3*m;}

else if (m>=1536) {mt=5*m−2048;}

else {mt=4*m−512;}

Cf=(e<<10)+(mt>>3)

This interpolation is simple to implement in hardware, and is aconsiderably closer approximation to a logarithmic interpolation thaninterpolating the integer interpretation of the bit pattern of afloating-point value.

The final value Cf is interpreted as an IEEE FP16 value. If the resultis +Inf or NaN, it is converted to the bit pattern 0x7BFF, which is thelargest representable finite value.

The index count is used in larger block sizes to indicate how manyindexes are actually present in the encoded block. This may be less thanthe size of the block, in which case the “missing” indexes have to bederived (as discussed above). (For example, a block encoding an 8×8texel array may only specify a 4×4 grid of indexes, in which case theremaining indexes will be generated using “index infill”, as discussedabove.)

Color Endpoint Modes

The format of the present embodiment supports 16 Color Endpoint Modes,which are described in more detail later. These endpoint modes aresubdivided into 4 classes:

class 0: one color endpoint pair is specified by 2 integers

class 1: one color endpoint pair is specified by 4 integers

class 2: one color endpoint pair is specified with 6 integers

class 3: one color endpoint pair is specified with 8 integers

Each of these classes contains 4 Color Endpoint Modes.

In 1-partition mode, the 4-bit Color Endpoint Mode field has thefollowing encoding:

Bits Usage 1:0 Endpoint Mode Class 3:2 Endpoint Mode within class

In modes with more than 1 partition, the color endpoint mode coding ismore elaborate:

First, we have a 2-bit Endpoint Mode Class Pair Selector; this selectoris encoded as follows:

Value Meaning 00 All endpoint pairs are of same type, this type follows01 All endpoint pairs are of class 0 or class 1 10 All endpoint pairsare of class 1 or class 2 11 All endpoint pairs are of class 2 or class3

If all endpoints are of same type, then this field is followed by a4-bit field, containing the Color Endpoint Mode used for all partitions.Otherwise, the field is followed by:

First, one bit per partition indicating which class its endpoint pairbelongs to.

Then, two bits per partition indicating which mode within the class itbelongs to.

Thus, for multi-partition modes, the endpoint mode representation maytake from 6 to 14 bits. Of these, the 6 first bits are stored just afterthe partition indexes, and the remaining bits are stored just below theindex bits (variable position).

This data layout ensures that the bits that indicate endpoint pair classalways appear in fixed locations; this helps decode performance inhardware.

Color Endpoint Representation

The color endpoints themselves are also represented using the IntegerSequence Encoding. The actual range being used is not directly encodedin the block; instead, the following is done:

-   -   From the partition-count and color-mode encodings, the number of        integers actually needed for the color encodings is computed.        This may be from 2 to 32, in increments of 2. (The lowest count,        2, occurs when using the Two-Luminance endpoint type with a        single partition; the highest count, 32, occurs when using the        Two-RGBA endpoint type with 4 partitions).    -   From the partition-count, color-mode encoding and        index-bits-mode, the number of bits needed to represent these        data fields is computed; this bit count is then subtracted from        the block size in order to obtain the number of bits actually        available for the color encodings.    -   Then, the largest range whose Integer Sequence Encoding will fit        into the available number of bits is determined (and used).        -   For example, if the space available for color endpoints is            35 bits, and the number of integers actually needed for the            color encodings is ten, then the range used will be 0 . . .            9: the Integer Sequence Encoding of ten integers of such a            range takes 34 bits, which fits. The next step up would be            to use the range 0 . . . 11; for this range, the Integer            Sequence Encoding would take 36 bits to encode ten integers,            which would not fit.            Integer Sequence Encoding

The Integer Sequence Encoding is a data encoding that is used to encodemost of the data in the compressed (encoded) texture data block in thepresent embodiment, and uses the integer encoding techniques of thetechnology described herein.

In order to use space efficiently, the encoding format is able to use anon-integer number of bits for its color endpoint and index fields. Thisis achieved by using trits (items that can take the values 0, 1, 2(whereas bits can only take the values 0 and 1)), and quints (which cantake the values 0, 1, 2, 3, 4). As trits and quints cannot berepresented directly in a binary computer the encoding format insteadstores trits and quints in a bit representation that allows n trits tobe represented with

$\lceil \frac{8n}{5} \rceil$bits and n quints to be represented with

$\lceil \frac{7n}{3} \rceil$bits.

The Integer Sequence Encoding is used to store a sequence of integerswithin a bounded range. The range used determines how many bits, tritsand quints are used to store each integer. The set of supported rangesand their bit/trit/quint allocation is:

Range Bits Trits/Quints 0 . . . 1 1 0 0 . . . 2 0 1 trit 0 . . . 3 2 0 0. . . 4 0 1 quint 0 . . . 5 1 1 trit 0 . . . 7 3 0 0 . . . 9 1 1 quint 0. . . 11 2 1 trit 0 . . . 15 4 0 0 . . . 19 2 1 quint 0 . . . 23 3 1trit 0 . . . 31 5 0 0 . . . 39 3 1 quint 0 . . . 47 4 1 trit 0 . . . 636 0 0 . . . 79 4 1 quint 0 . . . 95 5 1 trit 0 . . . 127 7 0 0 . . . 1595 1 quint 0 . . . 191 6 1 trit 0 . . . 255 8 0Encoding with Bits Only

If the range selected only uses bits, then integers are storedsequentially, with the lowest bit appearing first in the sequenceencoding. For example, if you want to encode a sequence of four numbers(a, b, c, d) whose range is 0 . . . 7 and whose bit-representation is(a=a₂a₁a₀, b=b₂b₁b₀, c=c₂c₁c₀, d=d₂d₁d₀), then the resulting sequence isthe 12-bit pattern d₂d₁d₀c₂c₁c₀b₂b₁b₀a₂a₁a₀

Encoding with Trits

If the range selected uses trits, then each integer is broken into twoparts before encoding: if the selected range has b bits, then the lowpart of a given integer x is given by L=X mod 2^(b) and the high part isgiven by

$H = {\lfloor \frac{X}{2^{b}} \rfloor.}$The L portion is represented by zero or more bits; the H portion isrepresented by one trit. The integers are then stored in groups of 5, asfollows:

-   -   First, a trit H is collected from every integer; this results in        5 trits. These are encoded into a trit-block; the full size of        the trit-block is 8 bits.

Then, bits are stored in the sequence in the following order:

-   -   First, the low bits for the first integer are stored.    -   Then, bits[1:0] of the trit-block are stored.    -   Then, the low bits for the second integer are stored.    -   Then, bits[3:2] of the trit-block are stored.    -   Then, the low bits for the third integer are stored.    -   Then, bit[4] of the trit-block is stored.    -   Then, the low bits for the fourth integer are stored.    -   Then bits[6:5] of the trit-block are stored.    -   Then, the low bits for the fifth integer are stored.    -   Then, bit [7] of the trit-block is stored.

This operation is repeated for every group of 5 integers, until all theintegers in the sequence have been consumed. At encode time, if thenumber of integers is not a multiple of 5, the integer sequence ispadded with 0s until its size becomes a multiple of 5. At decode time,if the number of integers to extract from the sequence is not a multipleof 5, then the sequence's bit representation has a (notionally infinite)string of zero-bits appended. This way, the format only stores bits forthe integers actually needed, instead of storing bits for amultiple-of-5 integers.

Decoding of a Trit-Block

Let the trit-block be denoted by b[7:0]. Now, proceed as follows:

First, we check whether b[4:2] is equal to 3′ b 1 1 1 . If it is,then: - Set c= { b[7:5], b[1:0]} - Set t₄= 2 and t₃= 2 Else - Set c=b[4:0] - If b[6:5]=2′ b 1 1 then □ Set t₄= 2 and t₃= { 1′ b 0 , b[7]} -Else □ Set t₄= { 1′ b 0, b[7]} and t₃= b[6:5] If c[1:0]= 2′ b 1 1 then -t₂= 2 , t₁= { 1′ b 0, c[4]} , t₀={c[3], c[2]&~c[3]} Else if c[3:2]= 2′ b1 1 then - t₂= 2 , t₁= 2 , t₀= c[1:0] Else - t₂ = { 1′ b 0, c[4]} , t₁=c[3:2] , t₀={c[1], c[0]&~c[1]}

-   -   This encoding is chosen based on two criteria:    -   It has the property that if only the n lowest trits are nonzero,        then only the

$\lceil \frac{8n}{5} \rceil$lowest bits of the trit-block can actually be nonzero.

The decoding process has a particularly efficient hardwareimplementation.

The AND-NOT operation on the lowest bit of t₀ ensures that thetrit-block unpacks to a tuple of 5 valid trits for all the 256 possibleinput values, even though there are only 3⁵=243 such tuples.

Example Integer Sequence with Trits

As an example, it will be assumed that 8 integers in the range 0 . . .11 are to be encoded using the Integer Sequence Encoding scheme of thepresent embodiment, and that these eight integers are {2, 9, 3, 5, 11,8, 0, 4}. First, the integers need to be split them into bits and trits;given that the 0 . . . 11 range has one trit and two bits, the result ofthis splitting is:

Trits (high part of the numbers): {0, 2, 0, 1, 2, 2, 0, 1}

Bits (low part of the numbers): {01, 01, 11, 01, 11, 00, 00, 00}

Given that there are 8 trits and 16 bits, the encoded Integer Sequencewill have

${16 + \lceil \frac{8*8}{5} \rceil} = {29\mspace{14mu}{{bits}.}}$

The trits now need to be encoded into two trit-blocks. The low 5 tritsare encoded into one trit-block; the high 3 trits are encoded into asecond trit-block.

Encoding with Quints

If the range selected uses quints, then each integer is broken into twoparts before encoding: if the selected range has b bits, then the lowpart of a given integer X is given by L=X mod 2^(b) and the high part isgiven by

$H = {\lfloor \frac{X}{2^{b}} \rfloor.}$The L portion is represented by zero or more bits; the H portion isrepresented by one quint. The integers are then stored in groups of 3,as follows:

-   -   First, a quint H is collected from every integer; this results        in 3 quints. These are encoded into a quint-block; the full size        of the quint-block is 7 bits.

Then, bits are stored in the sequence in the following order:

-   -   First, the low bits for the first integer are stored.    -   Then, bits[2:0] of the quint-block are stored.    -   Then, the low bits for the second integer are stored.    -   Then, bits[4:3] of the quint-block are stored.    -   Then, the low bits for the third integer are stored.    -   Then, bit[6:5] of the quint-block is stored.

This operation is repeated for every group of 3 integers, until all theintegers in the sequence have been consumed. At encode time, if thenumber of integers is not a multiple of 3, the integer sequence ispadded with 0s until its size becomes a multiple of 3. At decode time,if the number of integers to extract from the sequence is not a multipleof 3, then the sequence's bit representation has a (notionally infinite)string of zero-bits appended. This way, the format only stores bits forthe integers actually needed, instead of storing bits for amultiple-of-3 integers.

Decoding of a Quint-Block

Let the quint-block be denoted by b[6:0]. Now, proceed as follows:

If b[2:1]= 2′ b 1 1 and b[6:5]= 2′ b 0 0 then - Set t₂= {b[0], b[4]&~b[0], b[3]&~ b[0]}, t₁= 4 , t₀= 4 Else - If b[2:1]= 2′ b 1 1 then □ Sett₂= 4 and c={b[4:3], ~ b[6:5], b[0]} - Else □ Set t₂= { 1′ b 0, b[6:5]}and c=b[4:0] - If c[2:0]= 3′ b 1 0 1 then □ Set t₁= 4 and t₀= { 1′ b 0,c[4:3]} - Else □ Set t₁= { 1′ b 0, c[4:3]} and t₀= c[2:0]

This encoding is chosen by two criteria:

It has the property that if only the n lowest quints are nonzero, thenonly the

$\lceil \frac{7n}{3} \rceil$lowest bits of the quint-block can actually be nonzero.The decoding process has a particularly efficient hardwareimplementation.

The AND-NOT operation in the first rule ensures that all 128 possiblevalues decode to valid quint-triplets, even though there exists only5³=125 distinct quint-triplet values; four of the values (of the form7′b00xx111) represent the quint-triplet<4, 4, 4>.

The above decoding arrangement when using trits or quints effectively,for a stream of values, first emit the bits for each value, and thenemit sufficient bits from the packed trit- or quint-block to make up8n/5 (rounded up) bits or 7n/3 (rounded up) bits, respectively. Thisensures that the bitstream can be terminated after any value withoutlosing data.

The above trit/quint unpacking functions have a relatively low hardwarecost.

Other arrangements would, of course, be possible. For example, there area fairly large number of possible unpacking functions as such; some ofthese can be obtained by e.g. just inverting or swapping input or outputbits relative to what is described above; other ones can be obtained bymore elaborate sub-case approaches or by arithmetic (repeateddivision/modulo gives one particularly-easy-to-understand unpacking;however this approach is expensive in HW) or by look-up tables (whichallow arbitrary unpacking functions albeit at a higher cost).

Color Endpoint Unquantization

The color endpoints, after having been extracted from their IntegerSequence Encoding, need to be unquantized so that they end up in therange 0 . . . 255 instead of whatever range was used in the IntegerSequence Encoding.

For bit-only ranges, the unquantization is done using simple bitreplication.

In the case of a number composed of a trit/quint and one or more bits, amore elaborate procedure is performed:

First, the lowest bit b₀ is cut off.

-   -   Based on the range used, a constant C is selected; the trit or        quint is multiplied by this constant, resulting in the 9-bit        value T.    -   Based on the range used, a swizzle is performed on the remaining        bits; this 9-bit value is called B.    -   The addition T+B is then performed, then every bit of the        addition result is XORed with the bit b₀.    -   The result is a 9-bit number; b₀ is prepended to this number,        then the two lowest bits are discarded; this leaves 8 bits,        which is the unquantization result.

Below are tables that specify the swizzles and C values to use for thevarious ranges. Note that the lowest bit b₀ is not part of the inputbits.

Swizzles and C values for the case where a trit component is present:

Range Input bits Swizzle C 0 . . . 5 none 000000000 204 0 . . . 11 aa000a0aa0 93 0 . . . 23 ab ab000abab 44 0 . . . 47 abc abc000abc 22 0 .. . 95 abcd abcd000ab 11 0 . . . 191 abcde abcde000a 5

Swizzles and C values for the case where a quint component is present:

Range Input bits Swizzle C 0 . . . 9 none 000000000 113 0 . . . 19 aa0000aa00 54 0 . . . 39 ab ab0000aba 26 0 . . . 79 abc abc0000ab 13 0 .. . 159 abcd abcd0000a 6

This procedure produces an unquantization result with an error that isnever greater than off-by-1 relative to a correctly-roundingunquantization, while imposing a much lesser hardware cost (the“correctly rounding” unquantization procedure requires a multiplier,while the procedure presented here does not). It can have the sideeffect of scrambling the code point order, but this does not adverselyaffect image quality and is therefore considered acceptable (the encodercan easily compensate for this scrambling with a simple table lookup).

In this unquantisation procedure, the constant C is based on 1023/Mwhere M is the maximum value in the range, and is selected so that thetrit or quint makes a low-order contribution to the final unquantizedresult (while the bits make a high-order contribution, which is what theswizzle ultimately tries to achieve), such that the representablecodepoints are as evenly distributed as possible.

The swizzle patterns are related to the bit patterns of the reciprocalof M, so that the swizzle effectively does a truncated multiplication.

The manipulation using b₀ is done in order to obtain codepoint symmetry,so that if there exists a value that unquantizes to X, there also alwaysexists a value that unquantizes to 255-X. (This symmetry does not quitehold for the 0 . . . 2 and 0 . . . 4 ranges, which do not allow for theb₀ bit at all; these have an odd number of codepoints and thereforecannot be made symmetric.)

Color Endpoint Modes

The format of the present embodiment supports a total of 16 ColorEndpoint Modes; these modes control how the color endpoint integers areconverted into actual endpoint colors. The integers are the 0 . . . 255range integers that are present after the Color Endpoint Unquantization.

Several procedures are used repeatedly for several color conversionmodes; below, C++ implementations of these procedures are given:

The bit_transfer_signed Procedure

The bit_transfer procedure is used when one of the integers in anendpoint representation is deemed to require more precision than theother ones. It is used instead of independently assigning ranges to somesets of values, to skew the bit distribution more favourably.

Assuming two integers A and B, the bit-transfer works from A to B asfollows:

void bit_transfer_signed( uin8_t &a, uint8_t &b ) { b >>= 1; b |= a &0x80; a >>= 1; a &= 0x3F; if( (a & 0x20) != 0 ) a −= 0x40; }

Where necessary, the encoding should specify which values are the donorsand which the receivers of the transferred bits.

The Blue-Contraction Procedure

The blue_contract procedure is used to provide 1 extra bit of effectiveprecision for the red and green components of RGB colors that are closeto gray. The procedure works as follows:

void blue_contract( uint8_t &r, uint8_t &g, uint8_t &b ) { r = (r+b) >>1; g = (g+b) >> 1; }

This procedure is used, because the Applicants have recognised that ifthe texels in a block to be encoded are close to grey, then the endpointr, g, and b values will be close to one another, and it is advantageousin that case to encode the r and g components with more precision thanthe blue. The encoder may decide in this case to transfer precision fromthe blue by expanding the endpoint's green and red components accordingto the following blue-expansion transformation:G=(g<<1)−bR=(r<<1)−bB=b

(It can be determined that the endpoints are sufficiently close to thegray line by, for example, testing if the gray expansion transformresults in values that can be properly represented, i.e. they are stillin the range 0 . . . 1. Other arrangements would, of course, bepossible.)

The resulting R and G and B values are encoded as the endpoint values.

If this has been applied during encoding, the inverse “blue contraction”transformation described above must be applied to the endpoint valuesafter decoding:g=(G+B)>>1r=(R+B)>>1b=B

The encoder could use an additional bit to indicate to the decoder thatthis is required, but in the present embodiment it takes advantage ofthe fact that the order of endpoints is not important. A comparisonfunction between the two endpoint colors (e.g. by comparing the total ofr, g and b for each endpoint) is therefore defined. The encoder thenorders the endpoints such that that the results of the comparisonbetween the color values at endpoint 1 and endpoint 2 reflects whetherblue contraction should be applied during the decoding process or not.The decoder will then use the same comparison function to conditionallyapply blue contraction on the endpoint values after decoding (asdiscussed below).

Colour Endpoint Mode 0: Two Luminance or Alpha Endpoints

This mode takes as input two integers (v0, v1). If v0 is less than orequal to v1, then these integers form two RGBA colors(r0,g0,b0,a0)=(v0,v0,v0,0xFF) and (r1,g1,b1,a1)=(v1,v1,v1,0xFF).Otherwise, they form two RGBA colors (r0,g0,b0,a0)=(0,0,0,v1) and(r1,g1,b1,a1)=(0,0,0,v0).

Mode 1: Luminance, Base+Offset

This mode takes as input two integers (v0,v1). Two integers I0 and I1are then formed according to the following procedure:

void mode1_unpack( int v0, int v1, int &l0, int &l1 ) { l0 = (v0 >> 2) |(v1 & 0xC0); l1 = l0 + (v1 & 0x3f); if(l1 > 0xFF) l1 = 0xFF; }

After this, two RGBA colors are formed as (r0,g0,b0,a0)=(I0,I0,I0,0xFF)and (r1,g1,b1,a1)=(I1,I1,I1,0xFF)

Mode 2: HDR Luminance, Large Range

This mode takes as input two integers (v0,v1). These two integers arethen unpacked into a pair of HDR luminance values, as follows:

void mode2_unpack_y( int v0, int v1, int &y0, int &y1 ) { if(v1 >= v0) {y0 = (v0 << 4); y1 = (v1 << 4); } else { y0 = (v1 << 4) + 8; y1 = (v0 <<4) − 8; } }

This mode is intended for use when there are large luminance changes ina small region or there is a need to represent very large/smallluminance values.

Mode 3: HDR Luminance, Small Range

This mode takes as input two integers (v0,v1). These two integers arethen unpacked into a pair of HDR luminance values, as follows:

void mode3_unpack_y( int v0, int v1, int &y0, int &y1 ) { if((v0&0x80)!=0) { y0 = ((v1 & 0xE0) << 4) | ((v0 & 0x7F) << 2); d = (v1 & 0x1F) <<2; } else { y0 = ((v1 & 0xF0) << 4) | ((v0 & 0x7F) << 1); d = (v1 &0x0F) << 1; } y1 = y0 + d; if(y1 > 0xFFF) { y1 = 0xFFF; } }Mode 4: Two Luminance-Alpha Endpoints

This mode takes as input four integers (v0, v1, v2, v3). These integersform two RGBA colors (r0,g0,g0,a0)=(v0,v0,v0,v2) and(r1,g1,b1,a1)=(v1,v1,v1,v3)

Mode 5: Luminance-Alpha, Base+Offset

This mode takes as input four integers (v0, v1, v2, v3). From theseintegers, a base value (Ib, ab)=(v0, v2) and an offset value(Io,ao)=(v1,v3) are formed; the bit_transfer_signed procedure is thenperformed to transfer one bit from Io to Ib, and one bit from ao to ab;the two endpoints then form two RGBA colors as(r0,g0,b0,a0)=(Ib,Ib,Ib,ab) and (r1,g1,b1,a1)=(Ib+Io,Ib+Io,Ib+Io,ab+ao).The RGB values are clamped to the range 0x00 . . . 0xFF.

Mode 6: RGB and Scale

This mode takes as input four integers (v0, v1, v2, v3). From theseintegers, two endpoint colors are formed:

-   -   Endpoint color 0 is given by (r0,g0,b0,a0)=((v0*v3)>>8,        (v1*v3)>>8, (v2*v3)>>8, 0xFF)    -   Endpoint color 1 is given by (r1,g1,b1,a1)=(v0,v1,v2,0xFF)        Mode 7: Two HDR RGB Endpoints, Base and Scale

This mode takes as input four integers (v0, v1, v2, v3). These are acomplex packing allowing bits to be transferred from one color componentto another. The integers are unpacked into two HDR RGBA endpoint colorse0 and e1 as follows:

void mode7_unpack_y( int v0, int v1, color &e0, color &e1 ) { intmodeval = ((v0 & 0xC0) >> 6) | ((v1 & 0x80) >> 5) | ((v2 & 0x80) >> 4);int majcomp; int mode; if( (modeval & 0xC ) != 0xC ) { majcomp =modeval >> 2; mode = modeval & 3; } else if( modeval != 0xF ) { majcomp= modeval & 3; mode = 4; } else { majcomp = 0; mode = 5; } int red = v0& 0x3f; int green = v1 & 0x1f; int blue = v2 & 0x1f; int scale = v3 &0x1f; int x0 = (v1 >> 6) & 1; int x1 = (v1 >> 5) & 1; int x2 = (v2 >> 6)& 1; int x3 = (v2 >> 5) & 1; int x4 = (v3 >> 7) & 1; int x5 = (v3 >> 6)& 1; int x6 = (v3 >> 5) & 1; int ohm = 1 << mode; if( ohm & 0x30 ) green|= x0 << 6; if( ohm & 0x3A ) green |= x1 << 5; if( ohm & 0x30 ) blue |=x2 << 6; if( ohm & 0x3A ) blue |= x3 << 5; if( ohm & 0x3D ) scale |= x6<< 5; if( ohm & 0x2D ) scale |= x5 << 6; if( ohm & 0x04 ) scale |= x4 <<7; if( ohm & 0x3B ) red |= x4 << 6; if( ohm & 0x04 ) red |= x3 << 6; if(ohm & 0x10 ) red |= x5 << 7; if( ohm & 0x0F ) red |= x2 << 7; if( ohm &0x05 ) red |= x1 << 8; if( ohm & 0x0A ) red |= x0 << 8; if( ohm & 0x05 )red |= x0 << 9; if( ohm & 0x02 ) red |= x6 << 9; if( ohm & 0x01 ) red |=x3 << 10; if( ohm & 0x02 ) red |= x5 << 10; static const int shamts[6] ={ 1,1,2,3,4,5 }; int shamt = shamts[mode]; red <<= shamt; green <<=shamt; blue <<= shamt; scale <<= shamt; if( mode != 5 ) { green = red -green; blue = red - blue; } if( majcomp == 1 ) swap( red, green ); if(majcomp == 2 ) swap( red, blue ); e1.r = clamp( red, 0, 0xFFF ); e1.g =clamp( green, 0, 0xFFF ); e1.b = clamp( blue, 0, 0xFFF ); e1.alpha =0x780; e0.r = clamp( red - scale, 0, 0xFFF ); e0.g = clamp( green -scale, 0, 0xFFF ); e0.b = clamp( blue - scale, 0, 0xFFF ); e0.alpha =0x780; }Mode 8: Two RGB Endpoints

This mode takes as input six integers (v0, v1, v2, v3, v4, v5). Fromthese integers, two sums: s0=(v0+v2+v4), s1=(v1+v3+v5) are computed.These two sums are then compared:

-   -   If s1>=s0, then the two endpoint colors are obtained as        (r0,g0,b0,a0)=(v0,v2,v4,0xFF) and (r1,g1,b1,a1)=(v1,v3,v5,0xFF)    -   If s1<s0, then the two endpoint colors are obtained as        (r0,g0,b0,a0)=(v1,v3,v5,0xFF) and (r1,g1,b1,a1)=(v0,v2,v4,0xFF);        both of these two endpoint colors are then subjected to the        blue_contraction procedure.        Mode 9: RGB Base+Offset

This mode takes as input six integers (v0, v2, v2, v3, v4, v5). Theseintegers form an RGB base (rb, gb, bb)=(v0, v2, v4) and an RGB offset(ro, go, bo)=(v1, v3, v5). The base and offset values are then modifiedby having the bit_transfer_signed procedure applied to them to move onebit from the offset to the base (that is, from ro to rb, from go to gband from bo to bb).

The two endpoint colors are then given by (rb,gb,bb,0xFF) and (rb+ro,gb+go, bb+bo, 0xFF).

If the offset sum s=(ro+go+bo) is negative, then the two endpointnumbers are swapped and have the blue_contraction procedure applied tothem). The RGB values are clamped to the range 0x00 . . . 0xFF.

Mode 10: RGB, Scale, and Two Alpha Endpoints

This mode takes as input six integers (v0, v1, v2, v3, v4, v5). First,use (v0,v1,v2,v3) to produce two endpoint colors just as in Mode 6. Thenreplace the alpha of the first endpoint color with v4 and the alpha ofthe second endpoint color with v5.

Mode 11: Two HDR RGB Endpoints

This mode takes as input six integers (v0, v1, v2, v3, v4, v5). Theseare a complex packing allowing bits to be transferred from one colorcomponent to another. The integers are unpacked into two HDR RGBAendpoint colors e0 and e1 as follows:

void mode11_unpack_rgb( int v0, int v1, int v2, int v3, int v4, int v5,color &e0, color &e1) { int majcomp = ((v4 & 0x80) >> 7) | ((v5 &0x80) >> 6); if( majcomp == 3 ) { e0 = (v0 << 4, v2 << 4, (v4 & 0x7f) <<5, 0x780); e1 = (v1 << 4, v3 << 4, (v5 & 0x7f) << 5, 0x780); return; }int mode = ((v1 & 0x80) >> 7) | ((v2 & 0x80) >> 6) | ((v3 & 0x80) >> 5);int va = v0 | ((v1 & 0x40) << 2); int vb0 = v2 & 0x3f; int vb1 = v3 &0x3f; int vc = v1 & 0x3f; int vd0 = v4 & 0x7f; int vd1 = v5 & 0x7f;static const int dbitstab[8] = {7,6,7,6,5,6,5,6}; vd0 = signextend( vd0,dbitstab[mode] ); vd1 = signextend( vd1, dbitstab[mode] ); int x0 =(v2 >> 6) & 1; int x1 = (v3 >> 6) & 1; int x2 = (v4 >> 6) & 1; int x3 =(v5 >> 6) & 1; int x4 = (v4 >> 5) & 1; int x5 = (v5 >> 5) & 1; int ohm =1 << mode; if( ohm & 0xA4 ) va |= x0 << 9; if( ohm & 0x08 ) va |= x2 <<9; if( ohm & 0x50 ) va |= x4 << 9; if( ohm & 0x50 ) va |= x5 << 10; if(ohm & 0xA0 ) va |= x1 << 10; if( ohm & 0xC0 ) va |= x2 << 11; if( ohm &0x04 ) vc |= x1 << 6; if( ohm & 0xE8 ) vc |= x3 << 6; if( ohm & 0x20 )vc |= x2 << 7; if( ohm & 0x5B ) vb0 |= x0 << 6; if( ohm & 0x5B ) vb1 |=x1 << 6; if( ohm & 0x12 ) vb0 |= x2 << 7; if( ohm & 0x12 ) vb1 |= x3 <<7; int shamt = (modeval >> 1 ) {circumflex over ( )} 3; va <<= shamt;vb0 <<= shamt; vb1 <<= shamt; vc <<= shamt; vd0 <<= shamt; vd1 <<=shamt; e1.r = clamp( va, 0, 0xFFF ); e1.g = clamp( va - vb0, 0, 0xFFF );e1.b = clamp( va - vb1, 0, 0xFFF ); e1.alpha = 0x780; e0.r = clamp( va -vc, 0, 0xFFF ); e0.g = clamp( va - vb0 - vc - vd0, 0, 0xFFF ); e0.b =clamp( va - vb1 - vc - vd1, 0, 0xFFF ); e0.alpha = 0x780; if( majcomp ==1 ) { swap( e0.r, e0.g ); swap( e1.r, e1.g ); } else if( majcomp == 2 ){ swap( e0.r, e0.b ); swap( e1.r, e1.b ); } } Unlike mode 7, this modeis able to represent the full HDR range.Mode 12: Two RGBA Endpoints

This mode takes as input eight integers (v0, v1, v2, v3, v4, v5, v6,v7). From these integers, two sums: s0=(v0+v2+v4), s1=(v1+v3+v5) arecomputed. These two sums are then compared:

-   -   If s1>=s0, then the two endpoint colors are obtained as        (r0,g0,b0,a0)=(v0,v2,v4,v6) and (r1,g1,b1,a1)=(v1,v3,v5,v7)    -   If s1<s0, then the two endpoint colors are obtained as        (r0,g0,b0,a0)=(v1,v3,v5,v7) and (r1,g1,b1,a1)=(v0,v2,v4,v6);        both of these two endpoint colors are then subjected to the        blue_contraction procedure.        Mode 13: RGBA Base+Offset

This mode takes as input eight integers (v0, v1, v2, v3, v4, v5, v6,v7). These integers form an RGBA base (rb, gb, bb, ab)=(v0,v2,v4,v6) andan RGB offset (ro, go, bo, ao)=(v1,v3,v5,v7). The bit_transfer_signedprocedure is then used to transfer a bit from the offset value to thebase values. The two endpoint colors are then given by (rb, gb, bb, ab)and (rb+ro, gb+go, bb+bo, ab+ao). If (ro+go+bo) is negative, then theblue_contraction procedure is applied to the RGB portion of eachendpoint.

Mode 14: Two HDR RGBA Endpoints with LDR Alpha

This mode takes as input eight integers (v0, v1, v2, v3, v4, v5, v6,v7). The RGB components are decoded from values (v0 . . . v5) in themanner of Mode 11 discussed above. The alpha components for endpoints 0and 1 are then filled in from values v6 and v7 respectively.

Mode 15: Two HDR RGBA Endpoints with HDR Alpha

This mode takes as input eight integers (v0, v1, v2, v3, v4, v5, v6,v7). The RGB components are decoded from values (v0 . . . v5) in themanner of Mode 11 discussed above. The alpha components are then decodedas follows from values v6 and v7 as follows:

void mode15_ unpack_alpha(int v6, int v6, int& alpha0, int& alpha1) {mode = ((v6 >> 7) & 1 ) | ((v7 >> 6) & 2); v6 &= 0x7F; v7 &= 0x7F;if(mode==3) { alpha0 = v6 << 5; alpha1 = v7 << 5; } else { v6 |= (v7 <<(mode+1))) & 0x780; v7 &= (0x3F >> mode); v7 {circumflex over ( )}=0x20 >> mode; v7 −= 0x20 >> mode; v6 <<= (4-mode); v7 <<= (4-mode); v7+= v6; v7 = clamp(v7, 0, 0xFFF); alpha0 = v6; alpha1 = v7; } }The Void-Extent Block

A Void-Extent block is an encoded texture data block that specifies aregion within the texture in which every texture data element should beallocated the same data value when decoded and in which every samplelook-up within the encoded texture will only use texture data elementshaving that same data value (in the present embodiment). If bits[8:0] ofthe compressed (encoded) block are “111111100”, then the compressedblock is a Void-Extent Block. This means that the block has a constantcolor that is common for all texels in the block, and the blockadditionally specifies a region within the texture (for a 2D block) inwhich every bilinear-sampled lookup within the texture will only touchtexels whose color is identical to this constant color.

The Void-Extent feature is intended to enable specific texturingoptimizations:

-   -   If a texture mapper uses a multipass method for trilinear        filtering or anisotropic mapping, it may use the information in        the Void-Extent block to ascertain that all its passes will only        ever access texels of the same value, and thus return that value        as the final texturing result immediately after the first pass        without running any further passes.    -   A texture mapper may additionally keep a cache of recently-seen        Void-Extent blocks and use them to suppress actual texture-cache        line fills from memory for subsequent texturing operations.    -   Using the Void-Extent information is not mandatory; a texture        mapper that does not implement these optimizations may ignore        the Void-Extent and just treat the block as a constant-color        block.        The following rules and observations apply:    -   If the Void-Extent coordinates are all 1s, then it is        interpreted as if the block has no Void-Extent at all and is        simply a constant-color block.        -   Encoders that cannot compute Void-Extents properly but still            wish to use constant-color blocks thus always have the            option to just specify an all-1s pattern for the Void-Extent            in order to produce a straight constant-color block.    -   If a Void-Extent appears in a mipmap other than the most        detailed (lowest) one, then the Void-Extent applies to all more        detailed (lower) mipmap levels as well. As such, a texture        mapper that implements mipmapping as a multipass method may        sample the least detailed (highest) mipmap first, then upon        encountering a Void-Extent, it may abstain from sampling the        more detailed (lower) mipmap.        -   A consequence of this rule is that if a block has a constant            color but the corresponding region in any of the more            detailed (lower) mipmaps do not have a constant color, then            the Void-Extent coordinates must be set to all 0s to signal            the absence of a Void-Extent block. This situation is always            the case for the top 1×1 level of any mipmap pyramid, and            may be the case for more detailed (lower) levels as well, in            case of e.g. checkerboard textures.    -   The constant-color itself is specified using IEEE-754-2008        compliant FP16 values; this is the way in the format of the        present embodiment to specify floating-point content that can        hold negative values.    -   If a Void-Extent extends all the way to the edge of a texture,        the filtered texturing result may not necessarily be equal to        the texel value specified in the Void-Extent block; this may        happen if data sources other than the texture surface itself        contributes to the final filtered result. In such cases, the        texture mapper must include such data into its filtering process        in the same manner as if the Void-Extent were not present.        Examples of such data sources are:        -   Texture border color, when the border color is different            from the color specified in the Void-Extent block.        -   Adjacent-face textures in case of Seamless Cube-Mapping        -   Neighboring texture repeat in the case of the “Repeat”            texture wrap mode    -   If the texture mapper is keeping a cache of recently-seen        Void-Extent blocks, it must guarantee that the presence of this        cache does not produce texture filtering results that are        different from the result it would have produced without the        cache; depending on the specifics of the filtering unit, this        may limit caching to Void-Extent blocks with very specific color        values (e.g. all components are 0 or 1).    -   The Void-Extent specified by a Void-Extent block does not need        to actually overlap the block itself; such non-overlap is        unlikely to be useful, though.    -   Invalid Void-Extents—that is, a Void-Extent specified across a        region of a texture that does not actually have a constant        color—will result in undefined texturing results.        2D Textures

For 2D textures, the Void-Extent Block has the following layout:

Bits Usage 8:0 “111111100” 9 Dynamic range flag 11:10 Reserved, set to“11”. 24:12 Void Extent: Low S coordinate 37:25 Void Extent: High Scoordinate 50:38 Void Extent: Low T coordinate 63:51 Void Extent: High Tcoordinate 79:64 Block color R component 95:80 Block color G component111:96  Block color B component 127:112 Block color A component

The Void Extent is defined by a (low, high) interval for the S and Ttexture coordinates. The interval endpoints are represented as UNORM13values; as such, to get normalized coordinates in the [0, 1] range, thevalues stored in the block must be divided by 2¹³−1.

The Dynamic Range flag indicates the format in which the block color isstored. A 0 indicates LDR colors, in which case the color components arestored as normalized 16-bit integer values. A 1 indicates HDR color, inwhich case the color components are stored as FP16 values.

3D Textures

For 3D textures, the Void-Extent Block has the following layout:

Bits Usage 8:0 “111111100” 9 Dynamic range flag 18:10 Void Extent: Low Scoordinate 27:19 Void Extent: High S coordinate 36:28 Void Extent: Low Tcoordinate 45:37 Void Extent: High T coordinate 54:46 Void Extent: Low Pcoordinate 63:55 Void Extent: High P coordinate 79:64 Block color Rcomponent 95:80 Block color G component 111:96  Block color B component127:112 Block color A component

The Void-Extent is defined by a (low, high) interval for the S, T and Ptexture coordinates. The interval endpoints are represented as UNORM9values; as such, to get normalized coordinates in the [0, 1] range, thevalues stored in the block must be divided by 2⁹−1.

The Dynamic Range flag indicates the format in which the block color isstored. A 0 indicates LDR colors, in which case the color components arestored as normalized 16-bit integer values. A 1 indicates HDR color, inwhich case the color components are stored as FP16 values.

Partitioning Pattern Generator

As discussed above, the encoding scheme of the present embodiment uses apartitioning pattern generator (a partitioning pattern generationfunction) in order to produce its partitioning patterns; this allows alarge number of partitioning patterns to be supported at minimalhardware cost. (This comes at a slight quality cost compared to usingoptimized partition tables, however this cost has been measured to beonly about 0.2 dB, which does not justify the large hardware cost ofproviding a full set of optimized tables.)

The generator itself is specified in the form of a C99 function. Thearguments to this function are:

-   -   a seed: this is the partition index specified at bits 17:8 in        the 128-bit compressed block. This seed may take values from 0        to 1023.    -   a partition count; this may be 2, 3 or 4.    -   x, y and z positions; these are x, y and z texel position        relative to the upper-left corner of the compressed block (for a        2D block, z is set to zero (0)).    -   a flag indicating small blocks; its value depends on the block        size being used. The value of this flag is chosen to be 1 if the        number of texels in the block is less than 31, otherwise it is        set to 0.

The function returns an integer value in the range 0 . . . 3 specifyingwhich partition the specified texel belongs to. The actual function isdefined as follows:

int select_partition( int seed, int x, int y, int z, int partitioncount,int small_block ) { // first, scale up coordinates for small blocks.if(small_block) { x <<= 1; y <<= 1; z <<= 1; } // then, compute eightpseudoranom numbers, all of uniform distribution. // They need to be atleast approximately statistically independent, // so that they can covera reasonably wide parameter space. // the random-seed is modified withthe partition-count, so that the // partitionings we generate for 2, 3and 4 partitions are distinct. seed += (partitioncount−1) * 1024; // weneed reproducibility of the pseudorandom numbers, which calls for // ahash function. The hash52( ) function is designed specifically toprovide // a strong pseudorandom distribution at a modest hardware cost.uint32_t rnum = hash52(seed); // construct the seed values from the hashvalue. While it is important that // the seeds are independent, it isnot important that they have great precision; // in fact, no errorimprovement was seen when using seeds wider than 4 bits. int seed1 =rnum & 0xF; int seed2 = (rnum >> 4) & 0xF; int seed3 = (rnum >> 8) &0xF; int seed4 = (rnum >> 12) & 0xF; int seed5 = (rnum >> 16) & 0xF; intseed6 = (rnum >> 20) & 0xF; int seed7 = (rnum >> 24) & 0xF; int seed8 =(rnum >> 28) & 0xF; int seed9 = (rnum >> 18) & 0xF; int seed10 =(rnum >> 22) & 0xF; int seed11 = (rnum >> 26) & 0xF; int seed12 =((rnum >> 30) | (rnum << 2)) & 0xF; // square the seeds. This biasesthem, so that they are more likely to // take small rather than largevalues. The seed values represent // frequencies for a 2D sawtoothfunction; squaring them causes // low frequencies to be more heavilyrepresented than high freqeuncies. // For the partition function, thiscauses partitionings with low frequencies // (large, cleanly-dividedregions) to appear more frequently than // partitionings with highfrequencies (lots of detail), while not entirely // blocking the latter.seed1 *= seed1; seed2 *= seed2; seed3 *= seed3; seed4 *= seed4; seed5 *=seed5; seed6 *= seed6; seed7 *= seed7; seed8 *= seed8; seed9 *= seed9;seed10 *= seed 10; seed11 *= seed11; seed12 *= seed 12; // performshifting of the seed values // this causes the sawtooth functions to getincreased high-frequency content along either // the X axis or the Yaxis or neither; the result is an increase in the amount of //partitionings that are dominated by horizontal/vertical stripes; theseare // relatively important for overall psnr. int sh1, sh2, sh3; // usethe bottom bit of the seed to toggle horiz/vert direction. if( seed & 1) { sh1 = (seed & 2 ? 4 : 5); sh2 = (partitioncount == 3 ? 6 : 5); }else { sh1 = (partitioncount == 3 ? 6 : 5); sh2 = (seed & 2 ? 4 : 5); }sh3 = (seed & 0x10) ? sh1 : sh2; seed1 >>= sh1; seed2 >>= sh2; seed3 >>=sh1; seed4 >>= sh2; seed5 >>= sh1; seed6 >>= sh2; seed7 >>= sh1;seed8 >>= sh2; seed9 >>= sh3; seed10 >>=sh3; seed11 >>=sh3;seed12 >>=sh3; // combine the seed values with the XYZ coordinates toproduce 3D planar functions // Each of them also has an offset added;this offset itself needs to be pseudorandom // and unbiased for optimalquality. Unlike the seeds themselves, this offset // needs to have auniform distribution. int a = seed1*x + seed2*y + seed11*z + (rnum >>14); int b = seed3*x + seed4*y + seed12*z + (rnum >> 10); int c =seed5*x + seed6*y + seed9*z + (rnum >> 6); int d = seed7*x + seed8*y +seed10*z + (rnum >> 2); // bitwise “AND” with a mask turns planarfunctions into sawtooth functions. a &= 0x3F; b &= 0x3F; c &= 0x3F; d &=0x3F; // remove some of the functions if we are using less than 4partitions. if( partitioncount < 4 ) d = 0; if( partitioncount < 3 ) c =0; // then, compare the resulting sawtooth-function values in order toselect // a partition. if( a >= b && a >= c && a >= d ) return 0; elseif( b >= c && b >= d ) return 1; else if( c >= d ) return 2; else return3; }

The generator relies on an auxiliary function called hash52( ); thisfunction itself is defined as follows:

// autogenerated hash function. This hash function was produced bygenerating // random instruction sequences (from the set: add-shift,xor-shift, multiply-by-odd-constant; // these operations have in commonthat they are all invertible and therefore cannot lose data) // and thenchecking whether the instruction sequence, when fed the input datasequence // 0,1,2,3, ... produces a good pseudorandom output datasequence. The randomness tests run // were George Marsaglia's “SomeDifficult-to-pass Tests Of Randomness”. // Several hundred sunchinstruction sequences were generated; “hash52” below was the // one thatappeared to have the most compact hardware representation. // themultiply-by-odd-constant steps had their constants specifically selectedso that they // could be implemented with three shift-add operations,which are much cheaper in hardware // than general multiplications.uint32_t hash52( uint32_t p ) { p {circumflex over ( )}= p >> 15; p *=0xEEDE0891; // (2{circumflex over ( )}4+1)*(2{circumflex over( )}7+1)*(2{circumflex over ( )}17−1) p {circumflex over ( )}= p >> 5; p+= p << 16; p {circumflex over ( )}= p >> 7; p {circumflex over ( )}=p >> 3; p {circumflex over ( )}= p << 6; p {circumflex over ( )}= p >>17; return p; }

Note that the arithmetic in hash52( ) must be implemented using unsignedintegers that are exactly 32 bits wide. Also note that the multiply maybe implemented as a series of three addition/subtraction operations.

The above partition generation function basically works by implementing2 to 4 sawtooth functions with pseudorandomly-selected directions andfrequencies; this is particularly cheap to implement in hardware whilebeing able to produce nearly all partition shapes of interest.

The seed (partition index) is used to generate the parameters for thesawtooth wave generation. As each different seed gives a differentcombination of waves, it can be thought of as a “pattern index”. (Theseed is accordingly, effectively equivalent to the index into thepattern table in a lookup-table based design.) A mask is used togenerate the sawtooth function. It effectively changes a continuouslyincreasing set of values (e.g. 0, 16, 32, 48, 64, 80, 96, 112, 128, 144,160 . . . ) into a repeating set. A mask of 0x3F applied to the previoussequence would give a sawtooth of (0, 16, 32, 48, 0, 16, 32, 48, 0, 16,32, 48, 0, 16, 32 . . . ). This is equivalent to the remainder whendividing by 64, but only works when the divisor is a power of two. It isalso very much cheaper to implement in hardware than a division circuit.

Other arrangements for determining the partitioning patterns could beused, if desired. For example, the function could be configured togenerate curved partitioning shapes. For example, x^2 and y^2 termscould be added into the sawtooth functions. This will yieldpartitionings with curved shapes (which the “basic” version of thesawtooth function is unable to provide). However, testing with actualcontent did not actually show any image quality improvement from theseshapes. This kind of curve support will also increase the hardware cost.

It would also be possible to use a ROM-based partition table, where thetable is, e.g., generated through an optimization process against largeamounts of game content. However, storing actual tables would consumelarge numbers of gates, which would also get multiplied by the number ofdifferent block sizes supported and could thereby hamper the scalabilityof the format.

FIG. 12 shows schematically an arrangement of a graphics processingsystem 20 that can use textures that have been encoded in accordancewith the present embodiment. In this embodiment, the graphics processingsystem 20 is a tile-based rendering system. However, other arrangementsare, of course, possible.

As shown in FIG. 12, the graphics processing system 20 includes a statemanagement system 21, a rasterising stage 22, and a rendering stage 23in the form of a rendering pipeline. It will be appreciated that each ofthe stages, elements, and units, etc., of the graphics processor 20 asshown in FIG. 12 may be implemented as desired and will accordinglycomprise, e.g., appropriate circuitry, and/or processing logic, etc.,for performing the necessary operation and functions.

The state management system 21 stores and controls state data and thestate of the graphics processing units to control the graphicsprocessing operation, as is known in the art.

The rasteriser 22 takes as its input primitives to be displayed, andrasterises those primitives to sampling positions and generatesfragments to be rendered, as is known in the art.

The rendering pipeline 23 takes fragments from the rasteriser 22 andrenders those fragments for display. As is known in the art, therendering pipeline 23 will include a number of different processingunits, such as fragment shaders, blenders, texture mappers, etc.

The output from the rendering pipeline 23 (the rendered fragments) isoutput to tile buffers 24 (since the present embodiment is a tile-basedsystem). The tile buffers' outputs are then finally output to a framebuffer 25 for display.

FIG. 12 also shows schematically particular features of the graphicsprocessing system 20 that are provided in order for it to use texturesencoded in the manner of the present embodiment.

In particular, as shown in FIG. 12, the rendering pipeline 23 includes atexture mapping stage 26 configured to be able to access a texture listbuffer 27 to determine a texture needed for texturing a fragment that itreceives for rendering.

The texture list buffer 27 will indicate the texture that is required,and then, as is known in the art, the texture mapper 26 will fetch therelevant texture data from a memory 28 and used the fetched texture datato process the fragment in question.

The textures stored in the texture memory 28 are stored using theencoding format of the present embodiment. Thus, when the texture mapper26 needs a given texel or texels for applying to a fragment beingrendered, it will determine the texture map and encoded block withinthat map that it needs for the texel in question (e.g. based on theposition of the texel, as is known in the art), retrieve that block fromthe memory 28 and then determine the texel's value (e.g. colours) fromthe encoded block in the manner described above.

The texture mapper 26 includes a suitable decoder (decoding circuitry)to do this. This decoder may, e.g., be in the form of a dedicatedhardware element that is configured to decode textures encoded in theform of the present embodiment, or it may, e.g., comprise programmableprocessing circuitry that has been programmed appropriately to be ableto decode textures encoded in the form of the present embodiment. In anembodiment a dedicated hardware decoder is used.

In the present embodiment, the decoding process comprises firstdetermining whether the position of a texture data element to be decodedis within a previously encountered and stored constant data value region(void extent). If it is, the stored corresponding constant data valuefor the constant data value region (void extent) in question is thenreturned as the data value to use for the texture data elementimmediately (i.e. without accessing and decoding any encoded texturedata block).

On the other hand, if the position of a texture data element to bedecoded is not within a previously encountered and stored constant datavalue region (void extent), then it is determined which encoded texturedata block in the set of encoded texture data blocks representing thetexture map to be decoded represents (contains) the texel whose value isrequired (i.e. that is to be decoded). This is done based on theposition of the texel and knowledge of the block size and size of thetexture. The identified encoded texture data block is then accessed(e.g. fetched) from memory.

It is then determined whether the encoded block is a Void Extent block(i.e. indicates a constant data value region) or not, by determiningwhether the block contains the void extent flag (and also that the blockis not simply a “constant colour” block).

If the block is a Void Extent block, the decoder determines the constantdata value indicated in the Void Extent block and uses that value as thedata value for the texel in question.

The decoder also determines from the encoded Void Extent block theextent of the constant data value region specified by the block, andstores that information together with the constant data value for theregion (void extent) in question for future use. This information isstored for the most recent Void Extent blocks that the decoder hasaccessed, on a first-in, first out basis. Other arrangements would, ofcourse, be possible.

The decoding process in an embodiment also comprises, where the encodedtexture data block to be decoded is indicated (flagged) as being a VoidExtent block, then determining from the information in the blockindicating the extent of the constant data value region whether theblock is a true “Void Extent” block, or whether it is in fact only a“constant colour” block. In the latter case, it is in an embodiment alsodetermined whether the constant data value applies to more detailedmipmaps or not. If the block is a “constant colour” block, the decoderdetermines the constant data value indicated in the block and uses thatvalue as the data value for the texel in question.

Where the encoded texture data block is not a Void Extent or a “constantcolour” block (i.e. is a “normal” block), the decoder determines thevalue for the texel from the encoded block of texture data as follows:

1. Find the x,y,z position of the texel to be decoded, relative to thecorner of the block (for 2D blocks, z=0).

2. If there is more than one partition, pass the x,y,z position and theseed (partition index) through the partition generation function todetermine the partition number for the texel.

3. Read and decode the endpoint values for the partition selected instep 2. This process depends on the colour endpoint mode.

4. Read and decode the index for the texel. Where to find the indexdata, and how to decode it, is determined by the index range, indexcount, and number of index planes.

5. Interpolate between the endpoint colors using the index value, asspecified above.

6. If there are two index planes, repeat steps 4-5 for the second index,and combine the color components from the separate planes (e.g. RGB fromone, A from another into a single RGBA value).

7. The final color is the decoded texel color.

Thus, in the present embodiment, the decoding process for a given texelwhose value is required will comprise the following steps:

Determine the position of the texel being looked up

-   -   If it is inside a cached void-extent        -   return the constant colour value for that extent immediately    -   else        -   calculate which block the texel is in        -   load the block        -   if the block is a constant-colour block            -   return the constant colour value            -   if its a void extent block                -   cache the void extend bounds and the colour        -   if its not a constant-colour block            -   decode as normal

This is repeated for each texel value that is required, and theso-generated, decoded texel values are then applied to samplingpositions (fragments) that are being rendered to generate rendered datafor those sampling positions (fragments), which rendered data is then,e.g., written to the frame buffer for a display to display the“textured” sampling positions and/or fragments.

As discussed above, the decoder (the texture mapping process) is alsoconfigured, in response to recognition of a “constant data value” regionindicating Void Extent block to: not perform (avoid) subsequent passesin a multi-pass texture mapping process once such a block has beenidentified; not sample (avoid sampling) more detailed mipmaps in amulti-pass mip-mapping process once such a constant data value regionindicating block has been identified; cache recently loaded/processedconstant data value region indicating (Void Extent) blocks and use themto suppress (texture) cache filling from memory for subsequent decoding(texturing) operations; and/or not load (avoid loading) adjacent encodedtexture data blocks, where a constant data value region indicating (VoidExtent) block has been recognised.

As will be appreciated from the above, in the decoding arrangements, theactual data values (e.g. in terms of their format and what theyrepresent) that are generated for the set of data values to be used fora texture data block and for the individual texture data elements willdepend on the nature of the texture data that is being encoded. Thus,for example, as discussed above, in the case of colour data and colourmaps, each data value will represent a given colour, and, e.g., comprisea set of colour values, such as RGB or RGBa values. On the other hand,for a luminance map, each data value may comprise and represent a singleluminance value. For normal-maps (bump maps), each data value willcomprise a set of components representing a normal vector, and forshadow maps (light maps), each data value will comprise and represent aset of values indicating, e.g., the presence or absence, and amount of,light or shadow, and so on.

The above primarily describes the decoding process used in theembodiment of the technology described herein. As will be appreciated bythose skilled in the art, the encoding process will be carried out in acorresponding converse manner.

Thus, to encode a given texture map using the above encoding format inthe present embodiment, the original texture map is first divided intoblocks of a selected size.

Each block of texture data elements is then tested to see whether theset of texture data elements of the block can be encoded as having thesame, constant data value. This is done by determining whether all thetexture data elements of the block have sufficiently similar data valuesto be encoded as a constant data value block (based, e.g., and in anembodiment, on some selected, in an embodiment predetermined, similaritymargin or threshold)

Where it is determined that the texture data elements of a block oftexture data elements to be encoded all have sufficiently similar datavalues, then the extent of a contiguous extended region within thetexture including the block in which every texture data element hassufficiently similar data values is determined. This is done byattempting to extend a rectangular (for 2D) or rectangular cuboid (for3D) region outwards from the edge of the block of texture data elementsin question (while still only including texture data elements havingsufficiently similar data (e.g. colour) values). Any suitable process,e.g. algorithm, can be used for this.

It should be noted here that the constant data value region does notneed to align with the boundaries of the blocks the original texture hasbeen divided into for encoding purposes, but can only partially cover oroverlap blocks that the original texture has been divided into.

If an extended “constant data value” region is found, then the block oftexture data elements in question is encoded as a Void Extent block,having the form discussed above.

The constant data value for an encoded Void Extent block may be selectedas desired, based on the value of the texture data elements in theoriginal texture in the region of the texture in question. For example,an average of the values of the texture data elements of the block (orvoid extent region) could be used as the constant data value for theencoded Void Extent texture data block. Other arrangements would, ofcourse, be possible.

It should be noted here that where a given block of texture dataelements is found to fall within a constant data value region in thetexture (and is encoded as such), that does not mean that other, e.g.adjacent, blocks of texture data elements that also fall within the sameconstant data value region do not need to be encoded. Rather, everyseparate block of texture data elements that falls within the sameconstant data value region (void extent) is still encoded as arespective separate encoded Void Extent texture data block specifyingthat region. This facilitates random access into the encoded texture.

The encoding process may also comprise identifying blocks of texturedata elements as being constant data value blocks but which do not alsospecify a greater constant data value region (as discussed above), ifdesired. These blocks of texture data elements should then be encoded as“constant colour” blocks having the form discussed above.

Where it is determined that the set of texture data elements of a blockof texture data elements don't all have sufficiently similar datavalues, then a “non-void extent” encoded texture data block representingthe block of texture data elements having the form discussed above isgenerated.

The encoding process for a “non-constant data value” block can becarried out in any suitable manner on or using the original texture datathat is to be encoded. For example, as in known prior art processes, theoriginal data for the block could be encoded using some or all of thevarious different encoding and partitioning possibilities that areavailable (i.e. that, in effect, a “non-constant data value” encodedtexture data block can represent). This will provide a set of possibleencoded blocks that can then be compared with the original data, so asto determine, e.g., which encoded version of the block gives the leasterror (on reproduction) when compared to the original data (whichencoding arrangement can then be selected as the one to use for thatoriginal texture data block when it is encoded).

This is done for each different block that the original data (e.g.texture map) has been divided into. The process may then be repeatedusing a different block size, and so on, if desired, until the blocksize and encoding arrangements giving the least error (or at least asufficiently small error) is found, which may then be selected as theencoding arrangement to use for the texture.

The original texture may then be encoded using the determined block sizeand the encoding arrangement determined for each block (or the alreadyencoded blocks from the testing used, if they have been retained), toproduce a stream or set of encoded texture data blocks representing, andcorresponding to, the original set of data (e.g. texture map). This setof encoded texture data blocks can then be stored, e.g. on a portablestorage device such as a DVD, for later use, e.g. when it is desired toapply the texture to an image to be rendered.

In an embodiment a set of mipmaps is generated to represent the texture,with each mipmap in an embodiment being generated in the above manner.Where mipmaps are used, the compression rate (and bit rate) is in anembodiment varied for (is different for) different mipmap levels, withhigher bit rates (i.e. lower levels of data compression) being used forsmaller mipmap levels (i.e. lower resolution mipmap levels).

Each block that the original data (e.g. texture map) is divided into isin an embodiment the same size and configuration. The block size that isbeing used is provided to the decoder. This may be done, for example, byincluding (indicating) the block size in a (global) data header that isassociated with (attached to) the set of encoded texture data blocks, orin any other suitable manner.

The selection algorithm can use any desired (and many different) testingschemes such as, for example, measuring the peak signal-to-noise ratiobetween the encoded version of a block and the original version of theblock.

The encoding can be carried out as desired, e.g. using a suitablyprogrammed general-purpose processor that, e.g., has access to theoriginal texture data in memory, or a suitable dedicated processor couldbe used.

Although the above embodiment has been described with reference totexture data in the form of colours, as discussed above, and as will beappreciated by those skilled in the art, the technology described hereinis also applicable to other forms of texture data, such asluminance-maps or bump-maps, etc., and to other, non-texture data. Insuch arrangements the data can be encoded or decoded in an advantageousmanner, but each data value will, e.g., represent a luminance value ornormal vector, etc., rather than a colour.

Similarly, although the present embodiment has been described primarilywith reference to the encoding of square or cubical blocks of texels,other texel block arrangements and configurations, such as the encodingof non-square rectangular blocks of texels and non-cubical rectangularcuboid blocks of texels would be possible, if desired.

FIGS. 3 to 11 illustrate the basic encoded block layouts that the formatof the present embodiment will produce. Each encoded block comprises, asdiscussed above, 128-bits.

FIG. 3 shows an overview of the basic block layout. Thus it shows theindex mode data in bits 0-10, the “partition count-1” data in bits11-12, and the filling of the remaining space with any necessary extraconfiguration data, and the respective endpoint colour data and texelindex data (which are both of variable width).

FIG. 4 shows the layout for a non-partitioned block. Thus in this case,the “partition-count-1” data in bits 11-12 is set to “00” and the colourendpoint mode data (shown as CEM in FIG. 4) is placed in bits 13-16.

FIG. 5 shows the layout for a non-partitioned block but which uses twoindex planes. In this case, as shown in FIG. 5, two bits are used to actas the colour component selector (CCS) for the second index plane. Thesebits appear immediately below the texel index data (which is variablewidth, as discussed above).

FIG. 6 shows the layout for a block encoding a block of texels (texturedata elements) that has been divided into two partitions. In this case,the encoded block includes, as shown in FIG. 6, the “partition count-1”“01” (as there are two partitions) in bits 11-12, and the partitionindex (seed) for the partitioning pattern generation function in bits13-22.

The encoded block also includes, as shown in FIG. 6, the colour endpointmode pair selector (CPS) value in bits 23-24, and colour endpoint modeindicators (information) for each of the two partitions. The colourendpoint mode information comprises a respective colour endpoint classbit Cn and 2-bit colour endpoint mode field CMn for each partition n,and is arranged such that all the colour class bits for all thepartitions are emitted first (in partition order), followed by thecolour mode fields for each partition (in order). If this all requiresmore than 6 bits, then the additional bits are stored just below thetexel index bits (which will be a variable position, as discussedabove). It can be more efficient for a hardware decoder for the colourclass bits to be at fixed positions in the encoded block.

Thus, as shown in FIG. 6, in the case of a two partition block, thecolour endpoint mode pair selector (CPS) value is placed in bits 23-24,the respective colour class bits, C0, C1, for each partition (the firstpartition, partition 0, and the second partition, partition 1,respectively) are first placed in bits 25-26 (i.e. after the colourendpoint mode pair selector bits), and the 2-bit colour endpoint modefields then follow (in partition order), up to the limit of 6-bits, withany remaining bits then being placed just below the texel index data.Thus, the colour endpoint mode indicator for the first partition(partition 0) is placed in bits 27-28 (CM0), and the colour endpointmode for the second partition (partition 1) is placed in bits 53-54(CM1). As shown, the additional bits required for the colour endpointmode for the second partition (CM1) appear immediately below the texelindex data. The block also includes appropriate sets of colour endpointdata for the two different partitions (endpoint colour data 0 andendpoint colour data 1, respectively).

FIG. 7 shows the layout for a block that encodes a block of texture dataelements that has been divided into three partitions. In this case, asshown in FIG. 7, there are three sets of colour endpoint class and modedata (C0, C1, C2, M (CM0), CM1 and CM2), one for each partition,arranged as discussed above in relation to FIG. 6, together withcorresponding sets of endpoint colour data (endpoint colour data 0,endpoint colour data 1 and endpoint colour data 2), one for eachpartition. In this case the two bits of CM0 (denoted by “M” in FIG. 7)are, as shown in FIG. 7, split between bit 28 and a variable positionimmediately below CM1. Also, as shown in FIG. 7, the “partition count-1”bits 11-12 are set accordingly to “10” to indicate that this is athree-partition block.

FIG. 8 shows the layout for a block that encodes a set of texture dataelements that have been divided into four partitions. In this case,there is accordingly four sets of colour endpoint class and mode dataand four sets of corresponding endpoint colour data, one for eachpartition. Also, as shown in FIG. 8, the “partition count-1” bits 11-12are set accordingly to “11” to indicate that this is a four-partitionblock.

FIG. 9 shows the layout for a block that encodes a set of texture dataelements that have been divided into two partitions and that also usestwo index planes. In this case, as shown in FIG. 9, the block includes acolour component selector (CCS) field for the second index plane. Inthis case, this colour component selector appears directly below theadditional colour endpoint mode bits (CM1) for the second partition,which are in turn directly below the texel index data bits. (The samelayout rule (scheme) applies to three and four partition blocks withdual index planes.)

FIG. 10 shows the layout for a 2D void-extent block (i.e. a blockindicating a constant colour value). Thus, as shown, bits 0 to 8 of theblock are set to indicate that the block is a void-extent block.

As shown in FIG. 10, the void-extent block includes data indicating theconstant colour for the block, and the extent over which that colourextends, in terms of low and high S and T values.

FIG. 11 shows the layout for a 3D void-extent block. This layoutessentially corresponds to the 2D void-extent block layout shown in FIG.10, but includes a further extent component P (as the encoded blockrepresents a 3D block of texture data elements).

The ability to use different indexing schemes, data generation schemes,partitioning arrangements, etc., using a common encoding format in thepresent embodiment, provides the ability to provide different levels ofcompression (i.e. to vary the bit rate (the number of bits used pertexture data element)) that is being used for a given encoded texturedata block. For example, by varying the block size being used, theindexing scheme, data generation scheme, and/or partitioningarrangement, etc., different levels of relative compression can beprovided for a given block encoding arrangement and/or for a giventexture map or maps, for example.

As will be appreciated from the above, the technology described herein,in its embodiments at least, includes a number of new and particularlyadvantageous features.

The encodings used for the trit-block and the quint-block and the methodused to interleave bits of trit/quint blocks with low-order bits(thereby providing tight bounds on the bit-counts of the IntegerSequence Encoding), provides enhanced efficiency.

The unquantization method for the color endpoints can take their valuesfrom 0 . . . N to 0 . . . 255 with rounding errors no greater than 1ulp, without using any multipliers.

The technology described herein, in its embodiments at least, providesand facilitates fine-grain adjustable resolution of both per-texelindexes and color-components. The resolution can be adjusted in stepsthat correspond to approximately ⅓ bits; this fine-grained adjustmentenables the format to trade off bits between indexes and color endpointson a per-block basis (e.g. in regions with smooth colors, one wouldallocate large numbers of bits to the endpoints and comparatively few tothe indexes; in regions with large amounts of detail, one would insteadspend comparatively fewer bits on the color endpoints and more bits onthe indexes, in order to represent the detail).

The resolution for color encoding is implicit and therefore does notneed to be encoded; instead, the resolution is picked based on thenumber of values required by the selected color endpoint types and thenumber of bits actually available for color encoding. The resolution isnormally picked as the highest resolution that will fit into the bitsactually available.

As can be seen from the above, the technology described herein provides,in its embodiments at least, a data compression format for use, e.g.,for texture maps in hardware graphics acceleration, that is inparticular suited for applications where random access into the encodeddata is desired. It can provide a high compression rate, whichaccordingly is particularly advantageous for portable and low powerdevices where power, bandwidth and storage space may be limited.Notwithstanding the relatively high compression rate and smaller compactnature of the data compression format of the technology describedherein, it is still very capable of compressing, for example, differentkinds of images, and in particular both real-world images and drawings,with little loss of quality.

Thus the technology described herein has particular application to themobile gaming market, since it provides a high compression rate andtherefore is suitable for devices with limited memory resources andmemory bandwidth. The high compression rate also facilitates, forexample, downloading of games or other applications, since it canalleviate network bandwidth and end user cost issues which increase orare related to the download time.

Furthermore, this can be achieved whilst still providing high imagequality (which, as is known in the art, is critical for small mobiledevices that have a limited screen size (as is known in the art, thesmaller the size and resolution of a screen, the more effect on theperceived image quality noise or other errors has)). The technique ofthe technology described herein can also be used for a wide range oftexture maps, such as including high contrast scenarios (e.g. drawings)and textures with alpha (transparency) values.

What is claimed is:
 1. A method of decoding a texture data block thatrepresents a set of texture data elements to be used in a graphicsprocessing system, the method comprising: determining from the texturedata block, by processing circuitry, a base-n value, where n is greaterthan two; using by the processing circuitry the base-n value todetermine an integer value encoded in the texture data block; and usingby the processing circuitry the integer value to determine a data valueof a texture data element that the texture data block represents.
 2. Themethod of claim 1, comprising: determining by the processing circuitrythe base-n value (n>2) from a bit representation included in the texturedata block that represents plural base-n values (n>2).
 3. The method ofclaim 1, further comprising: determining from an interleaved sequence ofbits in the texture data block, by the processing circuitry, bits of abit representation representing the values of plural base-n values (n>2)and bits representing the values of base-2 values encoded in the texturedata block; determining by the processing circuitry one or more of thebase-n values (n>2) and of the base-2 values from the determined bits;and determining by the processing circuitry one or more integer valuesencoded in the texture data block using a combination of the determinedbase-n values (n>2) and base-2 values.
 4. The method of claim 1, furthercomprising: determining by the processing circuitry a range that hasbeen used for encoding a set of integer values encoded in the encodedtexture data block based on the number of integer values in the set andthe space available in the encoded texture data block for encoding theset of integer values.
 5. The method of claim 1, further comprising:converting by the processing circuitry the encoded integer value fromits position within a range that was used for encoding the integervalue, to a corresponding position within a larger permitted range ofinteger values; and then using by the processing circuitry the resultingconverted integer value to determine a data value for a texture dataelement that the block represents.
 6. The method of claim 1, furthercomprising the processing circuitry assuming that any bits that aremissing from the encoded texture data block in a sequence of bitsencoding integer values are always zero.
 7. An apparatus for decoding anencoded texture data block that represents a set of texture dataelements to be used in a graphics processing system, the apparatuscomprising processing circuitry configured to use a base-n value, wheren is greater than two, included in the encoded texture data block todetermine an integer value encoded in the encoded texture data block;and uses the integer value to determine a data value of a texture dataelement that the texture data block represents.
 8. The apparatus ofclaim 7, further comprising: processing circuitry configured todetermine a base-n value (n>2) from a bit representation included in thetexture data block that represents plural base-n values (n>2).
 9. Theapparatus of claim 7, further comprising: processing circuitryconfigured to determine from an interleaved sequence of bits in anencoded texture data block, bits of a bit representation representingthe values of plural base-n values (n>2) and bits representing thevalues of plural base-2 values encoded in the encoded texture datablock; processing circuitry configured to determine a base-n value (n>2)and one or more base-2 values from the determined bits; and processingcircuitry configured to determine an integer value encoded in theencoded texture data block using a combination of the determined base-nvalue (n>2) and the determined base-2 value or values.
 10. The apparatusof claim 7, further comprising: processing circuitry configured todetermine a range that has been used for encoding a set of integervalues encoded in an encoded texture data block based on the number ofinteger values in the set and the space available in the encoded texturedata block for encoding the set of integer values.
 11. The apparatus ofclaim 7, further comprising: processing circuitry configured to convertan encoded integer value from its position within a range that was usedfor encoding the integer value, to a corresponding position within alarger permitted range of integer values; and processing circuitryconfigured to then use the resulting converted integer value todetermine a data value for a texture data element that the blockrepresents.
 12. The apparatus of claim 7, wherein the apparatus isconfigured to assume that any bits that are missing from an encodedtexture data block in a sequence of bits encoding integer values arealways zero.
 13. The apparatus of claim 7, wherein: the processingcircuitry is part of a graphics processor.
 14. A non-transitory computerreadable storage medium storing computer software code which whenexecuting on one or more processors performs a method of decoding atexture data block that represents a set of texture data elements to beused in a graphics processing system, the method comprising: determiningfrom the texture data block, by processing circuitry of the one or moreprocessors, a base-n value, where n is greater than two; using by theprocessing circuitry the base-n value to determine an integer valueencoded in the texture data block; and using by the processing circuitrythe integer value to determine a data value of a texture data elementthat the texture data block represents.